Allow callers of GitConfig to specify the pickle file path

This way we can put it in another directory than the config file
itself, e.g. hide it inside ".git" when parsing a ".gitmodules"
file from the working tree.

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

git_config.py

@@ -56,16 +56,20 @@ class GitConfig(object):
     return cls(file = os.path.join(gitdir, 'config'),
                defaults = defaults)
 
-  def __init__(self, file, defaults=None):
+  def __init__(self, file, defaults=None, pickleFile=None):
     self.file = file
     self.defaults = defaults
     self._cache_dict = None
     self._section_dict = None
     self._remotes = {}
     self._branches = {}
-    self._pickle = os.path.join(
-      os.path.dirname(self.file),
-      '.repopickle_' + os.path.basename(self.file))
+
+    if pickleFile is None:
+      self._pickle = os.path.join(
+        os.path.dirname(self.file),
+        '.repopickle_' + os.path.basename(self.file))
+    else:
+      self._pickle = pickleFile
 
   def Has(self, name, include_defaults = True):
     """Return true if this configuration file has the key.