grab: gracefully handle the case when we try to grab parent of wdir Before this patch, we go all the way down to obsmarker creation logic and gets an error saying that changeset cannot obsolete itself. We should be more smart here and error out early saying we cannot grab that changeset.

grab: move the initialization of pctx variable outside of if-else The pctx variable is used outside of the if-else but it was initialized under the conditional statements. Added test for the case when the initialization inside the if-else broke `hg grab --continue`.