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