builtin/replay: move core logic into "libgit.a"

builtin/replay: move core logic into "libgit.a"

Move the core logic used to replay commits into "libgit.a" so that it
can be easily reused by other commands. It will be used in a subsequent
commit where we're about to introduce a new git-history(1) command.

Note that with this change we have no sign-comparison warnings anymore,
and neither do we depend on `the_repository`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>