commands: use dirstateguard instead of begin/end-parentchange for backout Before this patch, "hg backout" uses 'begin'/'end'-'parentchange()' of 'dirstate' class to avoid writing incomplete dirstate changes out at failure. But this framework doesn't work as expected, if 'dirstate.write()' is invoked between them. In fact, in-memory dirstate changes may be written out at 'repo.status()' implied by 'merge.update()', even before this patch. To restore dirstate as expected at failure of "hg backout", this patch uses 'dirstateguard' instead of 'begin'/'end'-'parentchange()'.