builtin/replay: extract core logic to replay revisions

builtin/replay: extract core logic to replay revisions

We're about to move the core logic used to replay revisions onto a new
base into the "libgit.a" library. Prepare for this by pulling out the
logic into a new function `replay_revisions()` that:

  1. Takes a set of revisions to replay and some options that tell it how
     it ought to replay the revisions.

  2. Replays the commits.

  3. Records any reference updates that would be caused by replaying the
     commits in a structure that is owned by the caller.

The logic itself will be moved into a separate file in the next commit.
This change is not expected to cause user-visible change in behaviour.

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