builtin/replay: plug leaking `advance_name` variable

builtin/replay: plug leaking `advance_name` variable

The `advance_name` variable can either contain a static string when
parsed via the `--advance` command line option or it may be an allocated
string when set via `determine_replay_mode()`. Because we cannot be sure
whether it is allocated or not we just didn't free it at all, resulting
in a memory leak.

Split up the variables such that we can track the static and allocated
strings separately and then free the allocated one to fix the memory
leak.

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