pick: don't create any successors when there were no changes (issue6093)

For example, when a changeset that user is trying to pick was already grafted
or rebased (and no obsmarkers were created), pick will naturally say that
there's no changes to commit and will not create the new commit. In that case
it should not mark any changeset as a successor of the picked changeset,
because the actual file changes were already there before picking.

Technically, pick could (and did before this patch) mark the wdir commit as a
successor of picked changeset (may be useful in the most trivial cases when
e.g. wdir is a graft of the required change), but there are cases when the
picked change is introduced in an ancestor of wdir, or even in multiple
ancestors piece by piece. Pick definitely shouldn't be trying to guess
something this complicated, so with this patch it always marks the picked
change as pruned if it gave no changes to commit.

--HG--
branch : stable
2 jobs for topic/stable/pick-no-changes in 15 minutes and 6 seconds (queued for 8 minutes and 38 seconds)
Status Name Job ID Coverage
  Test
passed flake8 #29156

00:00:16

passed tests #29157

00:14:49