Implement a cURL stream

Handle binary DIFFABLEness properly

Stash workdir correctly when added in the index, modified in the workdir

Always set `GIT_DIFF_PATCH_DIFFABLE` for all files, regardless of
binary-ness, so that the binary callback is invoked to either
show the binary contents, or just print the standard "Binary files
differ" message.  We may need to do deeper inspection for binary
files where we have avoided loading the contents into a file map.

Ensure that even when we're forcing a binary diff that we do not
assume that there *is* a diff.  There should be an empty diff for
no change.

If the libcurl stream is available, use that as the underlying stream
instead of the socket stream. This allows us to set a proxy for HTTPS
connections.

We do not want libcurl to perform the TLS negotiation for us, so we
don't need to pass this option.

The TLS streams talk over the curl stream themselves, so we don't need
to ask for it explicitly. Do so in the case of the non-encrypted one so
we can still make use proxies in that case.

When linking against libcurl, use it as the underlying transport instead
of straight sockets. We can't quite just give over the file descriptor,
as curl puts it into non-blocking mode, so we build a custom BIO so
OpenSSL sends the data through our stream, be it the socket or curl
streams.

The information is exposed by curl for some crypto libraries in the form
of name:content strings. We can't do much more than return this
information.

Of the built-in ones, only cURL support it, but there's no reason a
user-provided stream wouldn't support it.

If the stream claims to support this feature, we can let the transport
set the proxy.

We also set HTTPPROXYTUNNEL option so curl can create a tunnel through
the proxy which lets us create our own TLS session (if needed).

cURL has a mode in which it acts a lot like our streams, providing send
and recv functions and taking care of the TLS and proxy setup for us.

Implement a new stream which uses libcurl instead of raw sockets or the
TLS libraries directly. This version does not support reporting
certificates or proxies yet.

We test the generation of the textual patch via the patch function,
which are just one of two possibilities to get the output.

Add a second patch generation via the diff function to make sure both
outputs are in sync.

Fixes

Fixed GIT_DELTA_CONFLICTED not returned in some cases

Add `const` qualifier

racy-git, the missing link

Check the repository version

When stashing the workdir tree, examine the index as well.  Using
a mechanism similar to `git_diff_tree_to_workdir_with_index`
allows us to determine that a file was added in the index and
subsequently modified in the working directory.  Without examining
the index, we would erroneously believe that this file was
untracked and fail to include it in the working directory tree.

Use a slightly modified `git_diff_tree_to_workdir_with_index` in
order to avoid some of the behavior custom to `git diff`.  In
particular, be sure to include the working directory side of a
file when it was deleted in the index.

Ensure that when a file is added in the index and subsequently
modified in the working directory, the stashed working directory
tree contains the actual working directory contents.

Drop `git_diff__merge_like_cgit_reversed`, since it's a copy and
paste mess of slightly incompatible changes.

This is something we do on re-init but not when opening a
repository. This hasn't particularly mattered up to now as the version
has been 0 ever since the first release of git, but the times, they're
a-changing and we will soon see version 1 in the wild. We need to make
sure we don't open those.

git will assume the repository format version is 0 if the value is not
there. Do the same.

If an index entry for a file that is not in HEAD is in conflicted state,
when diffing HEAD with the index, the status field of the corresponding git_diff_delta was incorrectly reported as GIT_DELTA_ADDED instead of GIT_DELTA_CONFLICTED.

This was due to handle_unmatched_new_item() initially setting the status
to GIT_DELTA_CONFLICTED but then overriding it later with GIT_DELTA_ADDED.

Explicitly handle GIT_DELTA_CONFLICTED in git_diff_merge()

This fixes a bug where if a file was in conflicted state in either diff,
it would not always remain in conflicted state in the merged diff.

CRLF

Perform LF->CRLF for core.autocrlf=true on non-Win32 because core
git does.

All platforms do terrible, horrible, no good, very bad translation
when core.autocrlf=true.  It's not just Windows!

Allow files to have mixed line endings instead of skipping processing
on them.

Use statistics (like core git) to control the behavior of the
to workdir CRLF filter.

Support hierarchical test resource data, such that you can have
`tests/resources/foo/bar` and move the `bar` directory in as
a fixture.

Calling `cl_fixture_sandbox` on a path that is not directly beneath
the test resources directory succeeds, placing that directory into
the test fixture.  (For example, `cl_fixture_sandbox("foo/bar")`
will sandbox the `foo/bar` directory as `bar`).

Add support for cleaning up directories created this way, by only
cleaning up the basename (in this example, `bar`) from the fixture
directory.

A corpus of files checked out with Git (Linux, 1.9.1) to ensure that
produce identical data when checking out using a CRLF filter.

A corpus of files checked out with Git for Windows (2.4.1.windows.1)
to ensure that we produce identical data when checking out using a
CRLF filter.