GitLab

Previously there were no presubmit checks to verify that all watchlist
entries are valid e-mails. Bogus watchlist entries break git-cl-upload.
The new check will ensure that WATCHLISTS contains only valid e-mails.

Change-Id: I5d808327029fafa945582f5abba500eaacb500ba
Reviewed-on: https://chromium-review.googlesource.com/c/1325086
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606298}