We are going to rework this function, but before adding changes let's
move it to more appropriate place. Since xcommit_tree() creates commit
object from tree and parents and is pretty standard git function - the
appropriate place is gitobjects.

NOTE we cannot just replace xcommit_tree() with g.CreateCommit() as the
    latter works with already loaded tree and parent objects, but we
    want to be able to make commits only knowing tree and parents sha1.