shelve: add a method for deleting shelf to new shelf class

This is not necessary for my future changes, but it's more consistent
to encapsulate the knowledge of the various files in the `Shelf`
class.

Differential Revision: https://phab.mercurial-scm.org/D9742

mercurial/shelve.py

@@ -86,6 +86,10 b' class Shelf(object):'
     def open(repo, name):
         return Shelf(vfsmod.vfs(repo.vfs.join(shelvedir)), name)
 
+    @staticmethod
+    def open_backup(repo, name):
+        return Shelf(vfsmod.vfs(repo.vfs.join(backupdir)), name)
+
     def exists(self):
         return self.vfs.exists(self.name + b'.patch') and self.vfs.exists(
             self.name + b'.hg'
@@ -181,6 +185,10 b' class Shelf(object):'
                     self._backupfilename(backupvfs, filename),
                 )
 
+    def delete(self):
+        for ext in shelvefileextensions:
+            self.vfs.tryunlink(self.name + b'.' + ext)
+
 
 class shelvedstate(object):
     """Handle persistence during unshelving operations.
@@ -332,8 +340,7 b' def cleanupoldbackups(repo):'
         if mtime == bordermtime:
             # keep it, because timestamp can't decide exact order of backups
             continue
-        for ext in shelvefileextensions:
-            vfs.tryunlink(name + b'.' + ext)
+        Shelf.open_backup(repo, name).delete()
 
 
 def _backupactivebookmark(repo):