replay: extract logic to pick commits

replay: extract logic to pick commits

We're about to add a new git-history(1) command that will reuse some of
the same infrastructure as git-replay(1). To prepare for this, extract
the logic to pick a commit into a new "replay.c" file so that it can be
shared between both commands.

Rename the function to have a "replay_" prefix to clearly indicate its
subsystem.

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