amend: use dirstateguard instead of dirstate.invalidate Before this patch, "cmdutil.amend()" uses "dirstate.invalidate()" as a kind of "restore .hg/dirstate to the original status" during a failure. But it just discards changes in memory, and doesn't actually restore ".hg/dirstate". Then, it can't work as expected, if "dirstate.write()" is executed while processing. This patch uses "dirstateguard" instead of "dirstate.invalidate()" to restore ".hg/dirstate" at failure even if "dirstate.write()" is executed before failure. This is a part of preparations to fix the issue that the recent (in memory) dirstate isn't visible to external process (e.g. "precommit" hook).