strip: use a `changing_parents` context for --keep update

These are now properly scoped.

note: it would be neat to reuse this in `hg rollback`.

mercurial/strip.py

@@ -241,6 +241,7 b' def debugstrip(ui, repo, *revs, **opts):'
 
         revs = sorted(rootnodes)
         if update and opts.get(b'keep'):
+            with repo.dirstate.changing_parents(repo):
             urev = _findupdatetarget(repo, revs)
             uctx = repo[urev]