webkitpy: Small changes to merging script

Small changes to allow the merging script to work in the buildbot
environment.

BUG=524758

Review-Url: https://codereview.chromium.org/2759913002
Cr-Original-Commit-Position: refs/heads/master@{#458395}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d2908c0a13e525bffd180f9aa541078de4b9df8e
1 file changed