import: use dirstateguard instead of dirstate.invalidate
Before this patch, "commands.import()" 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 patch also removes "beginparentchage()" and "endparentchange()",
because "dirstateguard" makes them useless, too.
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).