Perform some error checking when examining symlink directories.

 Native Git allows symlinked directories under .git/refs. This
 change allows libgit2 to also look for references that live under
 symlinked directories.

Signed-off-by: Andy Doan <andy@opensourcefoundries.com>

FETCH_HEAD and multiple refspecs

We treat each refspec on its own, but the code currently overwrites the contents
of FETCH_HEAD so we end up with the entries for the last refspec we processed.

Instead, truncate it before performing the updates and append to it when
updating the references.

We want to do this in order to get FETCH_HEAD to be empty when we start updating
it due to fetching from the remote.

patch_parse: fix parsing unquoted filenames with spaces

Fix unpack double free

If an element has been cached, but then the call to
packfile_unpack_compressed() fails, the very next thing that happens is
that its data is freed and then the element is not removed from the
cache, which frees the data again.

This change sets obj->data to NULL to avoid the double-free. It also
stops trying to resolve deltas after two continuous failed rounds of
resolution, and adds a test for this.

Free OpenSSL peer certificate

libFuzzer: Prevent a potential shift overflow

cmake: let USE_ICONV be optional on macOS

Do not attempt to check out submodule as blob when merging a submodule modify/deltete conflict

Add Jonathan Tan to git.git-authors

Jonathan has consented via email to have his contributions to git reused in libgit2

diff_file: properly refcount blobs when initializing file contents

Per SSL_get_peer_certificate docs:
```
The reference count of the X509 object is incremented by one, so that it will not be destroyed when the session containing the peer certificate is freed. The X509 object must be explicitly freed using X509_free().
```

This makes it easier to cleanup allocated resources on exit.

libFuzzer: Fix missing trailer crash

When initializing a `git_diff_file_content` from a source whose data is
derived from a blob, we simply assign the blob's pointer to the
resulting struct without incrementing its refcount. Thus, the structure
can only be used as long as the blob is kept alive by the caller.

Fix the issue by using `git_blob_dup` instead of a direct assignment.
This function will increment the refcount of the blob without allocating
new memory, so it does exactly what we want. As
`git_diff_file_content__unload` already frees the blob when
`GIT_DIFF_FLAG__FREE_BLOB` is set, we don't need to add new code
handling the free but only have to set that flag correctly.

stransport: provide error message on trust failures

Fixes #4440

This change fixes an invalid memory access when the trailer is missing /
corrupt.

Found using libFuzzer.

libFuzzer: Fix a git_packfile_stream leak

The type of |base_offset| in get_delta_base() is `git_off_t`, which is a
signed `long`. That means that we need to make sure that the 8 most
significant bits are zero (instead of 7) to avoid an overflow when it is
shifted by 7 bits.

Found using libFuzzer.

This change ensures that the git_packfile_stream object in
git_indexer_append() does not leak when the stream has errors.

Found using libFuzzer.

Add git_status_file_at

openssl: fix thread-safety on non-glibc POSIX systems

diff_generate: fix unsetting diff flags

Use the same cert checking payload in WinHTTP

While the OpenSSL library provides all means to work safely in a
multi-threaded application, we fail to do so correctly. Quoting from
crypto_lock(3):

    OpenSSL can safely be used in multi-threaded applications provided
    that at least two callback functions are set, locking_function and
    threadid_func.

We do in fact provide the means to set up the locking function via
`git_openssl_set_locking()`, where we initialize a set of locks by using
the POSIX threads API and set the correct callback function to lock and
unlock them.

But what we do not do is setting the `threadid_func` callback. This
function is being used to correctly locate thread-local data of the
OpenSSL library and should thus return per-thread identifiers. Digging
deeper into OpenSSL's documentation, the library does provide a fallback
in case that locking function is not provided by the user. On Windows
and BeOS we should be safe, as it simply "uses the system's default
thread identifying API". On other platforms though OpenSSL will fall
back to using the address of `errno`, assuming it is thread-local.

While this assumption holds true for glibc-based systems, POSIX in fact
does not specify whether it is thread-local or not. Quoting from
errno(3p):

    It is unspecified whether errno is a macro or an identifier declared
    with external linkage.

And in fact, with musl there is at least one libc implementation which
simply declares `errno` as a simple `int` without being thread-local. On
those systems, the fallback threadid function of OpenSSL will not be
thread-safe.

Fix this by setting up our own callback for this setting. As users of
libgit2 may want to set it themselves, we obviously cannot always set
that function on initialization. But as we already set up primitives for
threading in `git_openssl_set_locking()`, this function becomes the
obvious choice where to implement the additional setup.

The macro `DIFF_FLAG_SET` can be used to set or unset a flag by
modifying the diff's bitmask. While the case of setting the flag is
handled correctly, the case of unsetting the flag was not. Instead of
inverting the flags, we are inverting the value which is used to decide
whether we want to set or unset the bits.

The value being used here is a simple `bool` which is `false`. As that
is being uplifted to `int` when getting the bitwise-complement, we will
end up retaining all bits inside of the bitmask. As that's only ever
used to set `GIT_DIFF_IGNORE_CASE`, we were actually always ignoring
case for generated diffs.

Fix that by instead getting the bitwise-complement of `FLAG`, not `VAL`.

In commit 9be638ec (git_diff_generated: abstract generated diffs,
2016-04-19), the code for generated diffs was moved out of the generic
"diff.c" and instead into its own module. During that conversion, it was
forgotten to remove the macros `DIFF_FLAG_IS_SET`, `DIFF_FLAG_ISNT_SET`
and `DIFF_FLAG_SET`, which are now only used in "diff_generated.c".

Remove those macros now.

CONTRIBUTING: add documentation of our commit message style