Fix dry_run run() return type to match check_output().

BUG=501173
R=dpranke@chromium.org, scottmg@chromium.org

Review URL: https://codereview.chromium.org/1183703003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295725 0039d316-1c4b-4281-b951-d872f2087c98

fetch.py

@@ -62,7 +62,7 @@ class Checkout(object):
   def run(self, cmd, **kwargs):
     print 'Running: %s' % (' '.join(pipes.quote(x) for x in cmd))
     if self.options.dry_run:
-      return 0
+      return ''
     return subprocess.check_output(cmd, **kwargs)