rerere forget path: forget recorded resolution

After you find out an earlier resolution you told rerere to use was a
mismerge, there is no easy way to clear it.  A new subcommand "forget" can
be used to tell git to forget a recorded resolution, so that you can redo
the merge from scratch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-rerere.c

@@ -110,6 +110,8 @@ int cmd_rerere(int argc, const char **argv, const char *prefix)
 
 	if (!strcmp(argv[1], "-h"))
 		usage(git_rerere_usage);
+	else if (!strcmp(argv[1], "forget"))
+		return rerere_forget(argv + 2);
 
 	fd = setup_rerere(&merge_rr);
 	if (fd < 0)