cmd/gopherbot: mention gerrit CLs on Github (cl2issue port)

This adds indexing of Github mentions to maintner, then adds gopherbot
functionality to say "CL mentions this issue."
on Github when a Gerrit CL references it.

Also the start of the cherry-pick milestone bot which needs the same
Github issue reference tracking. But that part's only barely started
and still disabled.

Fixes golang/go#18818
Updates golang/go#19776

Change-Id: Ie5f7f6845317a7c39cc36d7397c7539bf99c3f92
Reviewed-by: Brad Fitzpatrick <>
6 files changed