Talk about sharing objects and error messages; but the most important
part is about what to do with the cryptographic libraries, which sadly
have to become to responsibility of the application.

Extract the lock-setting functions into their own, as we cannot assume
that it's ok for us to set this unconditionally.

We're freeing the memory which holds the locks so we must make sure that
the locking function doesn't try to use 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