shallow.c: make check_shallow_file_for_update() static

No external callers exist.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

commit.h

@@ -254,7 +254,6 @@ extern int for_each_commit_graft(each_commit_graft_fn, void *);
 extern int is_repository_shallow(void);
 extern struct commit_list *get_shallow_commits(struct object_array *heads,
 		int depth, int shallow_flag, int not_shallow_flag);
-extern void check_shallow_file_for_update(void);
 extern void set_alternate_shallow_file(const char *path, int override);
 extern int write_shallow_commits(struct strbuf *out, int use_pack_protocol,
 				 const struct sha1_array *extra);

shallow.c

@@ -137,7 +137,7 @@ struct commit_list *get_shallow_commits(struct object_array *heads, int depth,
 	return result;
 }
 
-void check_shallow_file_for_update(void)
+static void check_shallow_file_for_update(void)
 {
 	if (is_shallow == -1)
 		die("BUG: shallow must be initialized by now");