Move print_commit_list to libgit.a

This is used by bisect.c, part of libgit.a while it stays in
builtin/rev-list.c. Move it to commit.c so that we won't get undefined
reference if a program that uses libgit.a happens to pull it in.

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

commit.h

@@ -222,4 +222,8 @@ struct commit *get_merge_parent(const char *name);
 
 extern int parse_signed_commit(const unsigned char *sha1,
 			       struct strbuf *message, struct strbuf *signature);
+extern void print_commit_list(struct commit_list *list,
+			      const char *format_cur,
+			      const char *format_last);
+
 #endif /* COMMIT_H */