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>

merge-recursive.h

@@ -59,9 +59,4 @@ struct tree *write_tree_from_memory(struct merge_options *o);
 
 int parse_merge_opt(struct merge_options *out, const char *s);
 
-/* builtin/merge.c */
-int try_merge_command(const char *strategy, size_t xopts_nr,
-		const char **xopts, struct commit_list *common,
-		const char *head_arg, struct commit_list *remotes);
-
 #endif