The default BUG_LINE_FORMAT is the existing BUG=%s. Projects that wish
to begin using Gerrit-style footers like Bug: %s can now set this in
codereview.settings.

BUG=616753

Change-Id: I4470311a86db228eab2a1655ae884736cce8c380
Reviewed-on: https://chromium-review.googlesource.com/451565
Commit-Queue: Mark Mentovai <mark@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>