While we do have a test which checks whether a written index of version
4 has the correct version set, we do not check whether this actually
enables path compression for index entries. This commit adds a new test
by adding a number of index entries with equal path prefixes to the
index and subsequently flushing that to disk. With suffix compression
enabled by index version 4, only the last few bytes of these paths will
actually have to be written to the index, saving a lot of disk space.
For the test, differences are about an order of magnitude, allowing us
to easily verify without taking a deeper look at actual on-disk
contents.

While we have a simple test to determine whether we can write an index
of version 4, we never verified that we are able to read this kind of
index (and in fact, we were not able to do so). Add a new repository
which has an index of version 4. This repository is then read from a new
test.

The init and cleanup functions for test suites are usually prepended to
our actual tests. The index::version test suite does not adhere to this
stile. Fix this.

In our code writing index entries, we carry around a `disk_size`
representing how much memory we have in total and pass this value to
`git_encode_varint` to do bounds checks. This does not make much sense,
as at the time when passing on this variable it is already out of date.
Fix this by subtracting used memory from `disk_size` as we go along.
Furthermore, assert we've actually got enough space left to do the final
path memcpy.

When using compressed index entries, each entry's path is preceded by a
varint encoding how long the shared prefix with the previous index entry
actually is. We currently encode a length of `(path_len - same_len)`,
which is doubly wrong. First, `path_len` is already set to `path_len -
same_len` previously. Second, we want to encode the shared prefix rather
than the un-shared suffix length.

Fix this by using `same_len` as the varint value instead.

We have a check in place whether the index has enough data left for the
required footer after reading an index entry, but this was only used for
uncompressed entries. Move the check down a bit so that it is executed
for both compressed and uncompressed index entries.

All index entry size computations are now performed in
`index_entry_size`. As such, we do not need the file-scope macros for
computing these sizes anymore. Remove them and move the `entry_size`
macro into the `index_entry_size` function.

Our code to write index entries to disk does not check whether the
entry that is to be written should use prefix compression for the path.
As such, we were overallocating memory and added bogus right-padding
into the resulting index entries. As there is no padding allowed in the
index version 4 format, this should actually result in an invalid index.

Fix this by re-using the newly extracted `index_entry_size` function.

Create a new function `index_entry_size` which encapsulates the logic to
calculate how much space is needed for an index entry, whether it is
simple/extended or compressed/uncompressed. This can later be re-used by
our code writing index entries.

The last written disk entry is currently being written inside of the
function `write_disk_entry`. Make behavior a bit more obviously by
instead setting it inside of `write_entries` while iterating all
entries.

To calculate the path of a compressed index entry, we need to know the
preceding entry's path. While we do actually set the first predecessor
correctly to "", we fail to update this while reading the entries.

Fix the issue by updating `last` inside of the loop. Previously, we've
been passing a double-pointer to `read_entry`, which it didn't update.
As it is more obvious to update the pointer inside the loop itself,
though, we can simply convert it to a normal pointer.

The index version 4 introduced compressed path names for the entries.
From the git.git index-format documentation:

    At the beginning of an entry, an integer N in the variable width
    encoding [...] is stored, followed by a NUL-terminated string S.
    Removing N bytes from the end of the path name for the previous
    entry, and replacing it with the string S yields the path name for
    this entry.

But instead of stripping N bytes from the previous path's string and
using the remaining prefix, we were instead simply concatenating the
previous path with the current entry path, which is obviously wrong.

Fix the issue by correctly copying the first N bytes of the previous
entry only and concatenating the result with our current entry's path.

When encoding varints to a buffer, we want to remain sure that the
required buffer space does not exceed what is actually available. Our
current check does not do the right thing, though, in that it does not
honor that our `pos` variable counts the position down instead of up. As
such, we will require too much memory for small varints and not enough
memory for big varints.

Fix the issue by correctly calculating the required size as
`(sizeof(varint) - pos)`. Add a test which failed before.

Fix issue with directory glob ignore in subdirectories

Submodule working directory

Introduce home directory expansion function for config files, attribute files

Fix proxy auto detect not utilizing callbacks

git_repository_set_head: use tag name in reflog

revparse: support open-ended ranges

To determine if a repository is a worktree or not, we currently check
for the existence of a "gitdir" file inside of the repository's gitdir.
While this is sufficient for non-broken repositories, we have at least
one case of a subtly broken repository where there exists a gitdir file
inside of a gitmodule. This will cause us to misidentify the submodule
as a worktree.

While this is not really a fault of ours, we can do better here by
observing that a repository can only ever be a worktree iff its common
directory and dotgit directory are different. This allows us to make our
check whether a repo is a worktree or not more strict by doing a simple
string comparison of these two directories. This will also allow us to
do the right thing in the above case of a broken repository, as for
submodules these directories will be the same. At the same time, this
allows us to skip the `stat` check for the "gitdir" file for most
repositories.

The check whether a repository is a worktree or not is currently done
inside of `git_repository_open_ext`. As we want to extend this function
later on, pull it out into its own function `repo_is_worktree` to ease
working on it.

The out-parameters of `find_repo` containing found paths of a repository
are a tad confusing, as they are not as obvious as they could be. Rename
them like following to ease reading the code:

- `repo_path` -> `gitdir_path`
- `parent_path` -> `workdir_path`
- `link_path` -> `gitlink_path`
- `common_path` -> `commondir_path`

The `path` out-parameter of `find_repo` is being sanitized initially
such that we do not try to append to existing content. The sanitization
is done via `git_buf_free`, though, which forces us to needlessly
reallocate the buffer later in the function. Fix this by using
`git_buf_clear` instead.

Fix GCC warnings

The fields `declared_size` and `received_bytes` of the `git_odb_stream`
are both of type `git_off_t` which is defined as a signed integer. When
passing these values to a printf-style string in
`git_odb_stream__invalid_length`, though, we format these as PRIuZ,
which is unsigned.

Fix the issue by using PRIdZ instead, silencing warnings on macOS.

The credentials callback reads the username and password via scanf into
fixed-length arrays. While these are simply examples and as such not as
interesting, the unchecked return value of scanf causes GCC to emit
warnings. So while we're busy to shut up GCC, we also fix the possible
overflow of scanf by using getline instead.

The `error` variable is used as a return value in the out-section of
both `odb_read_1` and `read_prefix_1`. While the value will actually
always be initialized inside of this section, GCC fails to realize this
due to interactions with the `found` variable: if `found` is set, the
error will always be initialized. If it is not, we return early without
reaching the out-statements.

Shut up the warnings by initializing the error variable, even though it
is unnecessary.

Update README: VS -> VSTS

libgit2 is no longer used in Visual Studio Team Services, it's used in
Visual Studio Team Services.

Support '..' and '...' ranges where one side is not specified.
The unspecified side defaults to HEAD.

Closes #4223

The current signature of `git_worktree_prune` accepts a flags field to
alter its behavior. This is not as flexible as we'd like it to be when
we want to enable passing additional options in the future. As the
function has not been part of any release yet, we are still free to
alter its current signature. This commit does so by using our usual
pattern of an options structure, which is easily extendable without
breaking the API.

When creating a new worktree, we do have a potential race with us
creating the worktree and another process trying to delete the same
worktree as it is being created. As such, the upstream git project has
introduced a flag `git worktree add --locked`, which will cause the
newly created worktree to be locked immediately after its creation. This
mitigates the race condition.

We want to be able to mirror the same behavior. As such, a new flag
`locked` is added to the options structure of `git_worktree_add` which
allows the user to enable this behavior.

tests: repo: fix repo discovery tests on overlayfs

Debian and Ubuntu often use schroot to build their DEB packages in a
controlled environment. Depending on how schroot is configured, our
tests regarding repository discovery break due to not being able to find
the repositories anymore. It turns out that these errors occur when the
schroot is configured to use an overlayfs on the directory structures.

The reason for this failure is that we usually refrain from discovering
repositories across devices. But unfortunately, overlayfs does not have
consistent device identifiers for all its files but will instead use the
device number of the filesystem the file stems from. So whenever we
cross boundaries between the upper and lower layer of the overlay, we
will fail to properly detect the repository and bail out.

This commit fixes the issue by enabling cross-device discovery in our
tests. While it would be preferable to have this turned off, it probably
won't do much harm anyway as we set up our tests in a temporary location
outside of the parent repository.