Before trying to rtransform using the given refspec to figure out what
the name of the upstream branch is on the remote, we must make sure that
the target of the refspec applies to the current branch's upstream.

When we update FETCH_HEAD we check whether the remote is the current
branch's upstream remote. The code does not check whether the current
refspec is relevant for this reference but always tries to perform the
reverse transformation, which causes it to error out if the refspec
doesn't match the reference.

Thanks to Pierre-Olivier Latour for the reproduction recipe.

Since it's not necessarily obvious, mention the merging state and how to
clear it.

revert tests: always use core.autocrlf=false in tests

Fixed the most obvious Clang static analyzer warnings

Flip THREADSAFE's default to ON

Threads are here to stay; and for a while now, users have had to call
the initialization function which sets up threads and crypto regardless
of whether the library was built threadsafe or not.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Rebase (merge)

Rename git_merge_head to git_annotated_commit, as it becomes used
in more operations than just merge.

Already cherry-picked commits should not be re-included.  If all changes
included in a commit exist in the upstream, then we should error with
GIT_EAPPLIED.

Commit the current patch of a rebase process.

`git_rebase_next` will apply the next patch (or cherry-pick)
operation, leaving the results checked out in the index / working
directory so that consumers can resolve any conflicts, as appropriate.

Abort an in-progress rebase and move the working directory and
repository back to the ORIG_HEAD state.

Introduce `git_rebase` to set up a rebase session that can
then be continued.  Immediately, only merge-type rebase is
supported.

Clean up some warnings