transaction: accept a 'location' argument for registertmp

This will allow generation of temporary files outside of store. This will be
useful for bookmarks.

mercurial/transaction.py

@@ -225,13 +225,13 b' class transaction(object):'
         self._backupsfile.flush()
 
     @active
-    def registertmp(self, tmpfile):
+    def registertmp(self, tmpfile, location=''):
         """register a temporary transaction file
 
         Such file will be delete when the transaction exit (on both failure and
         success).
         """
-        self._addbackupentry(('', '', tmpfile, False))
+        self._addbackupentry((location, '', tmpfile, False))
 
     @active
     def addfilegenerator(self, genid, filenames, genfunc, order=0,