Add --prune option to fetch when syncing a mirror repo

When syncing a mirror repo, add the --prune option to the fetch
command to force removal of stale refs from the mirror.

Change-Id: I4b43b2a5c86b9915627887c16f6569066f3ab978

project.py

@@ -1836,6 +1836,8 @@ class Project(object):
       cmd.append('--quiet')
     if not self.worktree:
       cmd.append('--update-head-ok')
+    if self.manifest.IsMirror:
+      cmd.append('--prune')
     cmd.append(name)
 
     # If using depth then we should not get all the tags since they may