repair: make strip() return backup file path

narrowhg wants to strip some commits and then re-apply them after
applying another bundle. Having repair.strip() return the bundle path
will be helpful for it.

mercurial/repair.py

@@ -244,6 +244,9 b' def strip(ui, repo, nodelist, backup=Tru'
             vfs.unlink(tmpbundlefile)
 
     repo.destroyed()
+    # return the backup file path (or None if 'backup' was False) so
+    # extensions can use it
+    return backupfile
 
 def rebuildfncache(ui, repo):
     """Rebuilds the fncache file from repo history.