Backport fixes to v0.24

When the array is empty `cmp` never gets set by the comparison
function. Initialize it so we return ENOTFOUND in those cases.

Only provide the empty tree internally, which matches git's behavior.
If we provide the empty blob then any users trying to write it with
libgit2 would omit it from actually landing in the odb, which appear
to git proper as a broken repository (missing that object).

The `PKG_CHECK_MODULES` function searches a pkg-config module and
then proceeds to set various variables containing information on
how to link to the library. In contrast to the `FIND_PACKAGE`
function, the library path set by `PKG_CHECK_MODULES` will not
necessarily contain linking instructions with a complete path to
the library, though. So when a library is not installed in a
standard location, the linker might later fail due to being
unable to locate it.

While we already honor this when configuring libssh2 by adding
`LIBSSH2_LIBRARY_DIRS` to the link directories, we fail to do so
for libcurl, preventing us to build libgit2 on e.g. FreeBSD. Fix
the issue by adding the curl library directory to the linker
search path.

According to the reference the git_checkout_tree and git_checkout_head
functions should accept NULL in the opts field

This was broken since the opts field was dereferenced and thus lead to a
crash.

When calling `http_connect` on a subtransport whose stream is already
connected, we first close the stream in case no keep-alive is in use.
When doing so, we do not reset the transport's connection state,
though. Usually, this will do no harm in case the subsequent connect
will succeed. But when the connection fails we are left with a
substransport which is tagged as connected but which has no valid
stream attached.

Fix the issue by resetting the subtransport's connected-state when
closing its stream in `http_connect`.

The .gitignore file allows for patterns which unignore previous
ignore patterns. When unignoring a previous pattern, there are
basically three cases how this is matched when no globbing is
used:

1. when a previous file has been ignored, it can be unignored by
   using its exact name, e.g.

   foo/bar
   !foo/bar

2. when a file in a subdirectory has been ignored, it can be
   unignored by using its basename, e.g.

   foo/bar
   !bar

3. when all files with a basename are ignored, a specific file
   can be unignored again by specifying its path in a
   subdirectory, e.g.

   bar
   !foo/bar

The first problem in libgit2 is that we did not correctly treat
the second case. While we verified that the negative pattern
matches the tail of the positive one, we did not verify if it
only matches the basename of the positive pattern. So e.g. we
would have also negated a pattern like

    foo/fruz_bar
    !bar

Furthermore, we did not check for the third case, where a
basename is being unignored in a certain subdirectory again.

Both issues are fixed with this commit.

When failing to initialize a new stransport stream, we try to
release already allocated memory by calling out to
`git_stream_free`, which in turn called out to the stream's
`free` function pointer. As we only initialize the function
pointer later on, this leads to a `NULL` pointer exception.

Furthermore, plug another memory leak when failing to create the
SSL context.

The `SSLCopyPeerTrust` call can succeed but fail to return a trust
object if it can't load the certificate chain and thus cannot check the
validity of a certificate. This can lead to us calling `CFRelease` on a
`NULL` trust object, causing a crash.

Handle this by returning ECERTIFICATE.

Don't try to determine when sysdirs are uninitialized.  Instead, simply
initialize them all at `git_libgit2_init` time and never try to
reinitialize, except when consumers explicitly call `git_sysdir_set`.

Looking at the buffer length is especially problematic, since there may
no appropriate path for that value.  (For example, the Windows-specific
programdata directory has no value on non-Windows machines.)

Previously we would continually trying to re-lookup these values,
which could get racy if two different threads are each calling
`git_sysdir_get` and trying to lookup / clear the value simultaneously.

According to git-fetch(1), "[t]he colon can be omitted when <dst>
is empty." So according to git, the refspec "refs/heads/master"
is the same as the refspec "refs/heads/master:" when fetching
changes. When trying to fetch from a remote with a trailing
colon with libgit2, though, the fetch actually fails while it
works when the trailing colon is left out. So obviously, libgit2
does _not_ treat these two refspec formats the same for fetches.

The problem results from parsing refspecs, where the resulting
refspec has its destination set to an empty string in the case of
a trailing colon and to a `NULL` pointer in the case of no
trailing colon. When passing this to our DWIM machinery, the
empty string gets translated to "refs/heads/", which is simply
wrong.

Fix the problem by having the parsing machinery treat both cases
the same for fetch refspecs.

And give it a default so that some compilers don't (unnecessarily)
complain.

Somehow I ended up with the following in my ~/.gitconfig:
[branch "master"]
remote = origin
merge = master
rebase = true

I assume something went crazy while I was running the git.git tests
some time ago, and that I never noticed until now.

This is not a good configuration, but it shouldn't cause problems. But
it does. Specifically, if you have this in your config, and you
perform the following set of actions:

create a remote
fetch from that remote
create a branch off of the remote master branch called "master"
delete the branch
delete the remote

The remote delete fails with the message "Could not find key
'branch.master.rebase' to delete". This is because it's iterating over
the config entries (including the ones in the global config) and
believes that there is a master branch which must therefore have these
config keys.

https://github.com/libgit2/libgit2/issues/3856

When we create a blame origin, we try to look up the blob that is
to be blamed at a certain revision. When this lookup fails, e.g.
because the file did not exist at that certain revision, we fail
to create the blame origin and return `NULL`. The blame origin
that we have just allocated is thereby free'd with
`origin_decref`.

The `origin_decref` function does not only decrement reference
counts for the blame origin, though, but also for its commit and
blob. When this is done in the error case, we will cause an
uneven reference count for these objects. This may result in
hard-to-debug failures at seemingly unrelated code paths, where
we try to access these objects when they in fact have already
been free'd.

Fix the issue by refactoring `make_origin` such that we only
allocate the object after the only function that may fail so that
we do not have to call `origin_decref` at all. Also fix the
`pass_blame` function, which indirectly calls `make_origin`, to
free the commit when `make_origin` failed.

git only checks ceiling directories when its search ascends to a parent
directory.  A ceiling directory matching the starting directory will not
prevent git from finding a repository in the starting directory or a
parent directory.  libgit2 handled the former case correctly, but
differed from git in the latter case: given a ceiling directory matching
the starting directory, but no repository at the starting directory,
libgit2 would stop the search at that point rather than finding a
repository in a parent directory.

Test case using git command-line tools:

/tmp$ git init x
Initialized empty Git repository in /tmp/x/.git/
/tmp$ cd x/
/tmp/x$ mkdir subdir
/tmp/x$ cd subdir/
/tmp/x/subdir$ GIT_CEILING_DIRECTORIES=/tmp/x git rev-parse --git-dir
fatal: Not a git repository (or any of the parent directories): .git
/tmp/x/subdir$ GIT_CEILING_DIRECTORIES=/tmp/x/subdir git rev-parse --git-dir
/tmp/x/.git

Fix the testsuite to test this case (in one case fixing a test that
depended on the current behavior), and then fix find_repo to handle this
case correctly.

In the process, simplify and document the logic in find_repo():
- Separate the concepts of "currently checking a .git directory" and
  "number of iterations left before going further counts as a search"
  into two separate variables, in_dot_git and min_iterations.
- Move the logic to handle in_dot_git and append /.git to the top of the
  loop.
- Only search ceiling_dirs and find ceiling_offset after running out of
  min_iterations; since ceiling_offset only tracks the longest matching
  ceiling directory, if ceiling_dirs contained both the current
  directory and a parent directory, this change makes find_repo stop the
  search at the parent directory.

The MSYS2 build system automatically compiles all code with position-independent
code. When we manually add the -fPIC flag to the compiler flags, MSYS2 will
loudly complain about PIC being the default and thus not required.

Fix the annoyance by stripping -fPIC in MSYS2 enviroments like it is already
done for MinGW.

Indicate that if you make changes to libgit2 that you must distribute
the source _to libgit2_, not the source _of your program_.

The old pthread-file did re-implement the pthreads API with exact symbol
matching. As the thread-abstraction has now been split up between Unix- and
Windows-specific files within the `git_` namespace to avoid symbol-clashes
between libgit2 and pthreads, the rewritten wrappers have nothing to do with
pthreads anymore.

Rename the Windows-specific pthread-files to honor this change.

The function pthread_num_processors_np is currently unused and superseded by the
function `git_online_cpus`. Remove the function.

When no index file exists and a baseline is not explicitly provided, use
an empty baseline instead of trying to load `HEAD`.

`git_checkout_head` is sadly misunderstood as something that can
switch branches.  It cannot.  Update the documentation to reflect this.

The thread local storage is used to hold some global state that
is dynamically allocated and should be freed upon exit. On
Windows, we clean up the C run-time right after execution of
registered shutdown callbacks and before cleaning up the TLS.

When we clean up the CRT, we also cause it to analyze for memory
leaks. As we did not free the TLS yet this will lead to false
positives.

Fix the issue by first freeing the TLS and cleaning up the CRT
only afterwards.