Improve on the 'invalid object' error message at commit time

Not that anybody should ever get it, but somebody did (probably because
of a flaky filesystem, but whatever).  And each time I see an error
message that I haven't seen before, I decide that next time it will look
better.

So this makes us write more relevant information about exactly which
file ended up having issues with a missing object.  Which will tell
whether it was a tree object, for example, or just a regular file in the
index (and which one).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -329,7 +329,8 @@ static int update_one(struct cache_tree *it,
 			entlen = pathlen - baselen;
 		}
 		if (mode != S_IFGITLINK && !missing_ok && !has_sha1_file(sha1))
-			return error("invalid object %s", sha1_to_hex(sha1));
+			return error("invalid object %06o %s for '%.*s'",
+				mode, sha1_to_hex(sha1), entlen+baselen, path);
 
 		if (ce->ce_flags & CE_REMOVE)
 			continue; /* entry being removed */