strip: switch to mergestate.clean()

See the previous patches for why we're doing this.

hgext/strip.py

@@ -208,8 +208,7 b' def stripcmd(ui, repo, *revs, **opts):'
             repo.dirstate.write(repo.currenttransaction())
 
             # clear resolve state
-            ms = merge.mergestate(repo)
-            ms.reset(repo['.'].node())
+            merge.mergestate.clean(repo, repo['.'].node())
 
             update = False