prune: skip already obsolete commits

This makes `hg prune` skip already obsolete commits and just print a
message about them, just like how `hg rebase` does.

--HG--
branch : stable
7 jobs for topic/stable/prune-already-obsolete-for-stable in 5 minutes and 56 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed doc #105628

00:00:35

passed flake8 #105622

00:00:46

manual pytype #105623
allowed to fail manual
passed tests-py2-cext #105624

00:04:09

passed tests-py2-pure #105625

00:02:54

passed tests-py3-cext #105626

00:05:55

passed tests-py3-pure #105627

00:05:42