Refactor cache-updating code to eliminate unnecessary fetches.

git_cache.py

Index: git_cache.py
diff --git a/git_cache.py b/git_cache.py
index 92332808fe0758d46cd4a8170667280a16051926..5dac806dedadafdc40b10950849e09fb4c5c62f1 100755
--- a/git_cache.py
+++ b/git_cache.py
@@ -318,6 +318,7 @@ class Mirror(object):
 
   def populate(self, depth=None, shallow=False, bootstrap=False,
                verbose=False):
+    assert self.GetCachePath()
     if shallow and not depth:
       depth = 10000
     gclient_utils.safe_makedirs(self.GetCachePath())
@@ -428,6 +429,8 @@ class Mirror(object):
   @classmethod
   def UnlockAll(cls):
     cachepath = cls.GetCachePath()
+    if not cachepath:
+      return
     dirlist = os.listdir(cachepath)
     repo_dirs = set([os.path.join(cachepath, path) for path in dirlist
                      if os.path.isdir(os.path.join(cachepath, path))])