Enable tracing of ref scans and config unpickling

These are not as expensive as spawning a git command, but they are
not free either.  We want to keep track of how many times we wind
up calling them on any particular operation.

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

git_config.py

@@ -19,7 +19,7 @@ import re
 import sys
 from urllib2 import urlopen, HTTPError
 from error import GitError, UploadError
-from git_command import GitCommand
+from git_command import GitCommand, TRACE
 
 R_HEADS = 'refs/heads/'
 R_TAGS  = 'refs/tags/'
@@ -189,6 +189,8 @@ class GitConfig(object):
     except OSError:
       return None
     try:
+      if TRACE:
+        print >>sys.stderr, ': unpickle %s' % self.file
       return cPickle.load(open(self._pickle, 'r'))
     except IOError:
       os.remove(self._pickle)