Fix "local variable 'url' referenced before assignment" error.

scm.py

Index: scm.py
diff --git a/scm.py b/scm.py
index f0fa40fcb8b4500cd3084a13b3121ee6697227f8..67dd747d4d47938c823c58314b1697a87664ee9e 100644
--- a/scm.py
+++ b/scm.py
@@ -217,6 +217,7 @@ class GIT(object):
     # The -100 is an arbitrary limit so we don't search forever.
     cmd = ['git', 'log', '-100', '--pretty=medium']
     proc = gclient_utils.Popen(cmd, stdout=subprocess.PIPE)
+    url = None
     for line in proc.stdout:
       match = git_svn_re.match(line)
       if match: