mq: add an '-e/--exact' option to qpush This patch adds an '--exact/-e' option to qpush that will try to push the patches in the correct location in the DAG. Specifying this option does the following: * If --move is specified, abort. It makes no sense to move a patch to the front of the queue and try to apply it to its parent, because its parent is one of the patches we just moved it in front of! * If patches are already applied, abort. We don't want patch changesets scattered throughout the DAG. * If local changes are present, abort unless --force is used, as usual. * Find the first patch we're going to push (if we're pushing multiple patches with a target or --all). * If that patch doesn't have a parent, abort, obviously. * If the parent doesn't exist in the repo, abort. Something is wrong. * Update to the parent, then continue pushing the patches as normal.

dirstate: warn on invalid parents rather than aborting This allows more graceful recovery from some mangled dirstates

code style: prefer 'is' and 'is not' tests with singletons