Cache the per-user configuration to avoid duplicate instances

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

git_config.py

@@ -28,9 +28,13 @@ def IsId(rev):
 
 
 class GitConfig(object):
+  _ForUser = None
+
   @classmethod
   def ForUser(cls):
-    return cls(file = os.path.expanduser('~/.gitconfig'))
+    if cls._ForUser is None:
+      cls._ForUser = cls(file = os.path.expanduser('~/.gitconfig'))
+    return cls._ForUser
 
   @classmethod
   def ForRepository(cls, gitdir, defaults=None):