In the last runs, wpt-importer has failed to upload updates to import CLs.

Recent failure:
https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Fchromium.infra.cron%2Fwpt-importer%2F14%2F%2B%2Frecipes%2Fsteps%2Fupdate_wpt%2F0%2Fstdout

Log excerpt:
webkitpy.common.system.executive.ScriptError: Failed to run "['git', 'cl', 'upload', '-f', '-t', 'Update test expectations and baselines.', '--gerrit']" exit_code: 1

output: Last 500 characters of output:
A	third_party/WebKit/LayoutTests/platform/mac-mac10.11/http/tests/streams/readable-streams/general-expected.txt
A	third_party/WebKit/LayoutTests/platform/mac-mac10.9/http/tests/streams/readable-streams/general-expected.txt
A	third_party/WebKit/LayoutTests/platform/win/http/tests/streams/readable-streams/general-expected.txt
A	third_party/WebKit/LayoutTests/platform/win7/http/tests/streams/readable-streams/general-expected.txt


I've tried running `git cl upload -f -t "..." --gerrit`, and that command seems to be OK; it appears that it's not properly committing the changes before trying to upload the patchset, and so there are uncommitted changes at the time that git cl upload is run.