replay: introduce new builtin

replay: introduce new builtin

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 <redacted>
Co-authored-by: Christian Couder <redacted>
Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>