shelve: use listshelves() in cleanup function

The point of this patch is to make it so all the callers of
`shelvedfile.movetobackup()` look the same, so I can move it over to
the new `Shelf` class next.

Differential Revision: https://phab.mercurial-scm.org/D9710

mercurial/shelve.py

@@ -602,10 +602,11 b' def cleanupcmd(ui, repo):'
     """subcommand that deletes all shelves"""
 
     with repo.wlock():
-        for (name, _type) in repo.vfs.readdir(shelvedir):
-            suffix = name.rsplit(b'.', 1)[-1]
-            if suffix in shelvefileextensions:
-                shelvedfile(repo, name).movetobackup()
+        for _mtime, name in listshelves(repo):
+            for suffix in shelvefileextensions:
+                shfile = shelvedfile(repo, name, suffix)
+                if shfile.exists():
+                    shfile.movetobackup()
             cleanupoldbackups(repo)