If (hypothetically) you pointed your WebKit repo environment variable to
Blink and not WebKit, then the change_re will not find anything in the
log and the script will explode on changes[0] because changes=[].

Fix this by being robust to not being able to parse anything out of the
log in process_git_commits.

R=szager@chromium.org
BUG=none

Review URL: https://chromiumcodereview.appspot.com/16258010

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@205290 0039d316-1c4b-4281-b951-d872f2087c98