Let's refactor the code to handle a regular commit (a commit that is
neither a root commit nor a merge commit) into a single function instead
of keeping it inside cmd_replay().
This is good for separation of concerns, and this will help further work
in the future to replay merge commits.
Co-authored-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>