When we receive a packet of exactly four bytes encoding its
length as those four bytes it can be treated as an empty line.
While it is not really specified how those empty lines should be
treated, we currently ignore them and do not return an error when
trying to parse it but simply advance the data pointer.

Callers invoking `git_pkt_parse_line` are currently not prepared
to handle this case as they do not explicitly check this case.
While they could always reset the passed out-pointer to `NULL`
before calling `git_pkt_parse_line` and determine if the pointer
has been set afterwards, it makes more sense to update
`git_pkt_parse_line` to set the out-pointer to `NULL` itself when
it encounters such an empty packet. Like this it is guaranteed
that there will be no invalid memory references to free'd
pointers.

As such, the issue has been fixed such that `git_pkt_parse_line`
always sets the packet out pointer to `NULL` when an empty packet
has been received and callers check for this condition, skipping
such packets.

When adding a new entry to an existing index via `git_index_read_index`,
be sure to remove the tree cache entry for that new path.  This will
mark all parent trees as dirty.

Read a tree into an index, write the index, then re-open the index and
ensure that we are treesame to the original.

Update the flags to reset the path_len (to emulate `index_insert`)

Treat index entries with different modes as different, which they
are, at least for the purposes of up-to-date calculations.

Clear any error state upon each iteration.  If one of the iterations
ends (with an error of `GIT_ITEROVER`) we need to reset that error to 0,
lest we stop the whole process prematurely.

Read a tree into an index using `git_index_read_index` (by reading
a tree into a new index, then reading that index into the current
index), then write the index back out, ensuring that our new index
is treesame to the tree that we read.

typo fix

Don't generate conflicts when checking out a modified submodule and the
submodule is dirty or modified in the workdir.

Haiku and Hurd both pass extra bits in struct stat::st_mode.

0120000 is symbolic link, not commit

We compute offsets by executing `off |= (*delta++ << 24)` for
multiple constants, where `off` is of type `size_t` and `delta`
is of type `unsigned char`. The usual arithmetic conversions (see
ISO C89 §3.2.1.5 "Usual arithmetic conversions") kick in here,
causing us to promote both operands to `int` and then extending
the result to an `unsigned long` when OR'ing it with `off`.
The integer promotion to `int` may result in wrong size
calculations for big values.

Fix the issue by making the constants `unsigned long`, causing both
operands to be promoted to `unsigned long`.

An object's size is computed by reading the object header's size
field until the most significant bit is not set anymore. To get
the total size, we increase the shift on each iteration and add
the shifted value to the total size.

We read the current value into a variable of type `unsigned
char`, from which we then take all bits except the most
significant bit and shift the result. We will end up with a
maximum shift of 60, but this exceeds the width of the value's
type, resulting in undefined behavior.

Fix the issue by instead reading the values into a variable of
type `unsigned long`, which matches the required width. This is
equivalent to git.git, which uses an `unsigned long` as well.

When `git_repository__cvar` fails we may end up with a
`ignorecase` value of `-1`. As we subsequently check if
`ignorecase` is non-zero, we may end up reporting that data
should be removed when in fact it should not.

Err on the safer side and set `ignorecase = 0` when
`git_repository__cvar` fails.

When we read the header, we want to know the size and type of the
object. We're currently inflating the full delta in order to read the
first few bytes. This can mean hundreds of kB needlessly inflated for
large objects.

Instead use a packfile stream to read just enough so we can read the two
varints in the header and avoid inflating most of the delta.

Include any required threading libraries in our `libgit2.pc`.

openssl_read should return -1 in case of error.

SSL_read returns values <= 0 in case of error.

A return value of 0 can lead to an infinite loop, so the return value
of ssl_set_error will be returned if SSL_read is not successful (analog
to openssl_write).

While no extra header fields are defined for tags, git accepts them by
ignoring them and continuing the search for the message. There are a few
tags like this in the wild which git parses just fine, so we should do
the same.

Backport bug fixes to 0.24

Instead of hoping that we can get a racy entry by going real fast
and praying real hard, just create a racy entry.

When running as root, skip the unreadable file tests, because, well,
they're probably _not_ unreadable to root unless you've got some
crazy NSA clearance-level honoring operating system shit going on.

Remove the now-unnecessary entries vector.  Add `git_array_search`
to binary search through an array to accomplish this.

Take advantage of the constant size of tree-owned arrays and store them
in an array instead of a pool. This still lets us free them all at once
but lets the system allocator do the work of fitting them in.

Instead of copying over the data into the individual entries, point to
the originals, which are already in a format we can use.

If we're looking for a symlink, realpath will give us the resolved path,
which is not what we're after, but a canonicalized version of the path
the user asked for.

If we hit the EOF while trying to write a new value, it may be that
we're already in the section that we were looking for.  If so, do not
write a (duplicate) section header, just write the value.

We should notice that we are in the correct section to add. This is a
cosmetic bug, since replacing any of these settings does work.

When we turned strict object creation validation on by default, we
forgot to inform the refs::create tests of this.  They, in fact,
believed that strict object creation was off by default.  As a result,
their cleanup function went and turned strict object creation off for
the remaining tests.