allow deepening of a shallow repository

Now, by saying "git fetch -depth <n> <repo>" you can deepen
a shallow repository.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

commit.c

@@ -255,6 +255,19 @@ int write_shallow_commits(int fd, int use_pack_protocol)
 	return count;
 }
 
+int unregister_shallow(const unsigned char *sha1)
+{
+	int pos = commit_graft_pos(sha1);
+	if (pos < 0)
+		return -1;
+	if (pos + 1 < commit_graft_nr)
+		memcpy(commit_graft + pos, commit_graft + pos + 1,
+				sizeof(struct commit_graft *)
+				* (commit_graft_nr - pos - 1));
+	commit_graft_nr--;
+	return 0;
+}
+
 int parse_commit_buffer(struct commit *item, void *buffer, unsigned long size)
 {
 	char *tail = buffer;