For now, this is just a rename from `t/helper/test-fast-rebase.c` into
`builtin/replay.c` with minimal changes to make it build appropriately.
Let's add a stub documentation and a stub test script though.
Subsequent commits will flesh out the capabilities of the new command
and make it a more standard regular builtin.
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de> 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>