Improve repo sync performance by avoid git forks

By resolving the current HEAD and the manifest revision using pure
Python, we can in the common case of "no changes" avoid a lot of
git operations and directly jump out of the local sync method.

This reduces the no-op `repo sync -l` time for Android's 114 projects
from more than 6s to under 0.8s.

Signed-off-by: Shawn O. Pearce <sop@google.com>

git_refs.py

@@ -41,6 +41,17 @@ class GitRefs(object):
     except KeyError:
       return ''
 
+  def deleted(self, name):
+    if self._phyref is not None:
+      if name in self._phyref:
+        del self._phyref[name]
+
+      if name in self._symref:
+        del self._symref[name]
+
+      if name in self._mtime:
+        del self._mtime[name]
+
   def _NeedUpdate(self):
     for name, mtime in self._mtime.iteritems():
       try: