Fix thread lock around cachepath access.

R=cmp@chromium.org, agable@chromium.org
BUG=

Review URL: https://codereview.chromium.org/468253003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@291571 0039d316-1c4b-4281-b951-d872f2087c98

git_cache.py

@@ -200,7 +200,9 @@ class Mirror(object):
 
   @classmethod
   def SetCachePath(cls, cachepath):
+    cls.cachepath_lock.acquire()
     setattr(cls, 'cachepath', cachepath)
+    cls.cachepath_lock.release()
 
   @classmethod
   def GetCachePath(cls):