Move try_merge_command and checkout_fast_forward to libgit.a

These functions are called in sequencer.c, which is part of
libgit.a. This makes libgit.a potentially require builtin/merge.c for
external git commands.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>