As far as I know, this will involve adding an invocation of "git fetch origin/master" (probably using the Git class in webkitpy/common?) at https://cs.chromium.org/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/w3c/test_exporter.py. Or maybe this should be done immediately before calling exportable_commits_since() in export_first_exportable_commit()?

This is done automatically on the bot already.