Right now, if SHA1DC is disabled, the SHA1 backend is mostly chosen
based on which system libgit2 is being compiled on and which libraries
have been found. To give developers and distributions more choice,
enable them to request specific backends by passing in a
`-DSHA1_BACKEND=<BACKEND>` option instead. This completely replaces the
previous auto-selection.

Upstream git.git has changed their default SHA1 implementation to the
collision-detection algorithm SHA1DC in commit e6b07da27 (Makefile: make
DC_SHA1 the default, 2017-03-17). To match upstream, align ourselves and
switch over to SHA1DC by default.

winhttp: properly support ntlm and negotiate

Support using notes via a commit rather than a ref

Transfer fewer objects on push and local fetch

refs: traverse symlinked directories

Inflate large loose blobs

Ensure that we can recurse into directories via symbolic links.

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>

When parsing unauthorized responses, properly parse headers looking for
both NTLM and Negotiate challenges.  Set the HTTP credentials to default
credentials (using a `NULL` username and password) with the schemes
supported by ourselves and the server.

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

Writing very large files may be slow, particularly on inefficient
filesystems and when running instrumented code to detect invalid memory
accesses (eg within valgrind or similar tools).

Introduce `GITTEST_SLOW` so that tests that are slow can be skipped by
the CI system.

Teach the CommonCrypto hash mechanisms to support large files.  The hash
primitives take a `CC_LONG` (aka `uint32_t`) at a time.  So loop to give
the hash function at most an unsigned 32 bit's worth of bytes until we
have hashed the entire file.

Teach the win32 hash mechanisms to support large files.  The hash
primitives take at most `ULONG_MAX` bytes at a time.  Loop, giving the
hash function the maximum supported number of bytes, until we have
hashed the entire file.

Check the size of objects being read from the loose odb backend and
reject those that would not fit in memory with an error message that
reflects the actual problem, instead of error'ing later with an
unintuitive error message regarding truncation or invalid hashes.

Instead of paging to zlib in INT_MAX sized chunks, we can give it
as many as UINT_MAX bytes at a time.  zlib doesn't care how big
a buffer we give it, this simply results in fewer calls into zlib.

zlib will only inflate/deflate an `int`s worth of data at a time.
We need to loop through large files in order to ensure that we inflate
the entire file, not just an `int`s worth of data.  Thankfully, we
already have this loop in our `git_zstream` layer.  Handle large objects
using the `git_zstream`.

Introduce an internal API to get the object type based on a
length-specified (not null terminated) string representation.  This can
be used to compare the (space terminated) object type name in a loose
object.

Reimplement `git_object_string2type` based on this API.

Introduce a test for very large objects in the ODB.  Write a large
object (5 GB) and ensure that the write succeeds and provides us the
expected object ID.  Introduce a test that writes that file and
ensures that we can subsequently read it.

Introduce `git_prefixncmp` that will search up to the first `n`
characters of a string to see if it is prefixed by another string.
This is useful for examining if a non-null terminated character
array is prefixed by a particular substring.

Consolidate the various implementations of `git__prefixcmp` around a
single core implementation and add some test cases to validate its
behavior.

zlib will return `Z_BUF_ERROR` whenever there is more input to inflate
or deflate than there is output to store the result.  This is normal for
us as we iterate through the input, particularly with very large input
buffers.

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