shelve: move function for opening .patch file to new shelf class

The `opener()` method was used specifically for the `.patch` file, and
the new `Shelf` class deals with all files involved in a shelf, so I
renamed the function in the process.

Differential Revision: https://phab.mercurial-scm.org/D9705

mercurial/shelve.py

@@ -114,9 +114,6 b' class shelvedfile(object):'
     def stat(self):
         return self.vfs.stat(self.fname)
 
-    def opener(self, mode=b'rb'):
-        return self.vfs(self.fname, mode)
-
 
 class Shelf(object):
     """Represents a shelf, including possibly multiple files storing it.
@@ -193,6 +190,9 b' class Shelf(object):'
         finally:
             fp.close()
 
+    def open_patch(self, mode=b'rb'):
+        return self.vfs(self.name + b'.patch', mode)
+
 
 class shelvedstate(object):
     """Handle persistence during unshelving operations.
@@ -481,7 +481,7 b' def _shelvecreatedcommit(repo, node, nam'
     Shelf(repo, name).writeinfo(info)
     bases = list(mutableancestors(repo[node]))
     Shelf(repo, name).writebundle(bases, node)
-    with shelvedfile(repo, name, patchextension).opener(b'wb') as fp:
+    with Shelf(repo, name).open_patch(b'wb') as fp:
         cmdutil.exportfile(
             repo, [node], fp, opts=mdiff.diffopts(git=True), match=match
         )