[coverage] write commit coverage in "update" mode
[coverage] write commit coverage in "update" mode

Summary:
Write commit coverage in "update" mode to take advantage of
https://secure.phabricator.com/D14428 when it lands.  The extraneous parameter
should have no effect in the meantime.

Once our phabricator install is updated to include that diff, files in
Diffusion will display the latest available coverage.

Test Plan: n/a

Reviewers: guido

Reviewed By: guido

Subscribers: ddfisher, changesbot

Differential Revision: https://tails.corp.dropbox.com/D152377

dropbox/changes,dropbox/changes,dropbox/changes,dropbox/changes