SVN.GenerateDiff assumes SVN.Capture captures stderr.

TBR=dpranke@chromium.org
BUG=
TEST=


Review URL: http://codereview.chromium.org/7787008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@100270 0039d316-1c4b-4281-b951-d872f2087c98

scm.py

@@ -397,7 +397,8 @@ class SVN(object):
 
     Throws an exception if non-0 is returned.
     """
-    return subprocess2.check_output(['svn'] + args, **kwargs)
+    return subprocess2.check_output(
+        ['svn'] + args, stderr=subprocess2.PIPE, **kwargs)
 
   @staticmethod
   def RunAndGetFileList(verbose, args, cwd, file_list, stdout=None):