Right now it is only possible to create remotes from a repository. While
this is probably the most common use-case, there are commands which make
sense even without a repository, e.g. the equivalence of `git
ls-remote`. Add a new function `git_remote_create_detached`, which
simply accepts a URL.

There are only few actions which actually make sense for a detached
remote, like e.g. `git_remote_ls`, `git_remote_prune`. For all the other
actions, we have to report an error when the remote has no repository
attached to it. This commit does so and implements some tests.

The current error message when connecting to a remote when no URL is set
is missing information on whether it is missing a fetch or push URL.
Furthermore, it results in undefined behavior when using a remote
without name. Fix both issues.

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.

libssh2 shutdown

After calling `libssh2_init`, we need to clean up after the library by
executing `libssh2_exit` as soon as we exit. Register a shutdown handler
to do so which simply calls `libssh2_exit`. This fixes several memory
leaks.

We unconditionally return success when initializing libssh2, regardless
of whether `libgssh2_init` signals success or an error. Fix this by
checking its return code.

WIP: squash some memleaks

This reverts commit 55522376 and frees the snapshot properly.

Be sure to clean up looked up references.  Free buffers instead of
merely clearing them.  Use `git__free` instead of `free`.

Verify object hashes

transport: provide a getter for the proxy options

Debian HTTPS feature test failure

Do not free config when creating remote

Since this is allowed in `git_remote_upload`

The regression was introduced in 22261344

While the function reading an object from the complete OID already
verifies OIDs, we do not yet do so for reading objects from a partial
OID. Do so when strict OID verification is enabled.

The read_prefix_1 function has several return statements springled
throughout the code. As we have to free memory upon getting an error,
the free code has to be repeated at every single retrun -- which it is
not, so we have a memory leak here.

Refactor the code to use the typical `goto out` pattern, which will free
data when an error has occurred. While we're at it, we can also improve
the error message thrown when multiple ambiguous prefixes are found. It
will now include the colliding prefixes.

Verifying hashsums of objects we are reading from the ODB may be costly
as we have to perform an additional hashsum calculation on the object.
Especially when reading large objects, the penalty can be as high as
35%, as can be seen when executing the equivalent of `git cat-file` with
and without verification enabled. To mitigate for this, we add a global
option for libgit2 which enables the developer to turn off the
verification, e.g. when he can be reasonably sure that the objects on
disk won't be corrupted.

The upstream git.git project verifies objects when looking them up from
disk. This avoids scenarios where objects have somehow become corrupt on
disk, e.g. due to hardware failures or bit flips. While our mantra is
usually to follow upstream behavior, we do not do so in this case, as we
never check hashes of objects we have just read from disk.

To fix this, we create a new error class `GIT_EMISMATCH` which denotes
that we have looked up an object with a hashsum mismatch. `odb_read_1`
will then, after having read the object from its backend, hash the
object and compare the resulting hash to the expected hash. If hashes do
not match, it will return an error.

This obviously introduces another computation of checksums and could
potentially impact performance. Note though that we usually perform I/O
operations directly before doing this computation, and as such the
actual overhead should be drowned out by I/O. Running our test suite
seems to confirm this guess. On a Linux system with best-of-five
timings, we had 21.592s with the check enabled and 21.590s with the
ckeck disabled. Note though that our test suite mostly contains very
small blobs only. It is expected that repositories with bigger blobs may
notice an increased hit by this check.

In addition to a new test, we also had to change the
odb::backend::nonrefreshing test suite, which now triggers a hashsum
mismatch when looking up the commit "deadbeef...". This is expected, as
the fake backend allocated inside of the test will return an empty
object for the OID "deadbeef...", which will obviously not hash back to
"deadbeef..." again. We can simply adjust the hash to equal the hash of
the empty object here to fix this test.

We currently have no tests which check whether we fail reading corrupted
objects. Add one which modifies contents of an object stored on disk and
then tries to read the object.

The object::lookup tests do use the "testrepo.git" repository in a
read-only way, so we do not set up the repository as a sandbox but
simply open it. But in a future commit, we will want to test looking up
objects which are corrupted in some way, which requires us to modify the
on-disk data. Doing this in a repository without creating the sandbox
will modify contents of our libgit2 repository, though.

Create the repository in a sandbox to avoid this.

In the odb::backend::nonrefreshing test suite, we set up a fake backend
so that we are able to determine if backend functions are called
correctly. During the setup, we also parse an OID which is later on used
to read out the pseudo-object. While this procedure works right now, it
will create problems later when we implement hash verification for
looked up objects. The current OID ("deadbeef") will not match the hash
of contents we give back to the ODB layer and thus cannot be verified.

Make the hash configurable so that we can simply switch the returned for
single tests.

Branch renames with worktrees

socket_stream: continue to next addrinfo on socket creation failure

Honor read-only flag when writing to config backends

When connecting to a remote via socket stream, we first use getaddrinfo
to obtain the possible connection methods followed by creating and
connecting the socket. But when creating the socket, we error out as
soon as we get an invalid socket instead of trying out other address
hints returned by addrinfo.

Fix this by continuing on invalid socket instead of returning an error.
This fixes connection establishment with musl libc.

Configuration backends have a readonly-flag which is currently used to
distinguish configuration snapshots. But somewhat unexpectedly, we do
not use the flag to prevent writing to a readonly backend but happily
proceed to do so.

This commit modifies logic to also honor the readonly flag for
configuration setters. We will now traverse through all backends and
pick the first one which is not marked as read-only whenever we want to
write new configuration.

diff_parse: free object instead of its pointer

The config_file.h header provides some inline declarations accessing the
`git_config_backend`, but misses its declaration. Add the missing
include for "git2/sys/config.h" to add it.

In e7330016 (diff_parse: check return value of `git_diff_init_options`,
2017-03-20), we've introduced an error check whether we're able to
correctly initialize the diff options. This simple commit actually
introduced a segfault in that we now try to free the pointer to the
allocated diff in an error case, instead of the allocated diff itself.
This commit fixes the issue.

Misc fixes