Fix unused variable 'message' warning

Fix `git_commit_create` for an initial commit

`test_commit_commit__create_initial_commit_parent_not_current` was not correctly 
testing that `HEAD` was not changed. Now we grab the oid that it was pointing to
before the call to `git_commit_create` and the oid that it's pointing to afterwards
and compare those.

Rebase: rebase a branch with no merge base for in-memory

When calling `git_commit_create` with an empty array of `parents` and `parent_count == 0` 
the call will segfault at https://github.com/libgit2/libgit2/blob/master/src/commit.c#L107 
when it's trying to compare `current_id` to a null parent oid. 

This just puts in a check to stop that segfault.

odb: avoid inflating the full delta to read the header

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.

cmake: include threading libraries in pkg-config

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

Introduce `git_signature_from_buffer`

Fix return value of openssl_read (infinite loop)

Allow users to construct a signature from the type of signature
lines that actually appear in commits.

Annotated commits: differentiate between the ref names and the description

Differentiate between the ref_name used to create an annotated_commit
(that can subsequently be used to look up the reference) and the
description that we resolved this with (which _cannot_ be looked up).

The description is used for things like reflogs (and may be a ref name,
and ID something that we revparsed to get here), while the ref name must
actually be a reference name, and is used for things like rebase to
return to the initial branch.

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).

Test that we can properly abort a rebase when it is initialized by a
revspec.  This ensures that we do not conflate revspecs and refnames.

Remove traces of `git_blob_create_fromchunks`

Allow creating copies of `git_reference` objects.

Rebase improvements with IDs

tag: ignore extra header fields

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.

⚡ some warnings

When rebasing with IDs, we do not return to the `branch`,
we remain in a detached HEAD state.

When `init`ing a rebase from a detached HEAD, be sure to remember
that we were in a detached HEAD state so that we can correctly
`abort` the object that we just created.

Instead of `open`ing a rebase and `abort`ing that, test that we can
`abort` a rebase that has just begun with `init`.

Proxy configuration

We were downloading the jar from within an block which only runs for
MSVC. Move the download to the start of the test so it gets downloaded
for both.

Improve star-star matching

Instead of threading the state down to the larger loop, let's have the
loop where we detect the double star so each of them are easier to read.