receive-pack: allow deletion of corrupt refs

Occasionally, in some setups (*cough* forks on repo.or.cz *cough*) some
refs go stale, e.g. when the forkee rebased and lost some objects needed
by the fork.  The quick & dirty way to deal with those refs is to delete
them and push them again.

However, git-push first would first fetch the current commit name for the
ref, would receive a null sha1 since the ref does not point to a valid
object, then tell receive-pack that it should delete the ref with this
commit name.  delete_ref() would be subsequently be called, and check that
resolve_ref() (which does _not_ check for validity of the object) returns
the same commit name.  Which would fail.

The proper fix is to avoid corrupting repositories, but in the meantime
this is a good fix in any case.

Incidentally, some instances of "cd .." in the test cases were fixed, so
that subsequent test cases run in t/trash/ irrespective of the outcome of
the previous test cases.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

receive-pack.c

@@ -200,6 +200,10 @@ static const char *update(struct command *cmd)
 	}
 
 	if (is_null_sha1(new_sha1)) {
+		if (!parse_object(old_sha1)) {
+			warning ("Allowing deletion of corrupt ref.");
+			old_sha1 = NULL;
+		}
 		if (delete_ref(name, old_sha1)) {
 			error("failed to delete %s", name);
 			return "failed to delete";