cmd/gerritbot: don't query GitHub as much
cmd/gerritbot: don't query GitHub as much

Use maintner as a fast-path to determine whether comments are
duplicates or not.

Update golang/go#18517

Change-Id: I3ddc380e6e202371903ce62ae9e4c07e9805c1ed
Reviewed-on: https://go-review.googlesource.com/93435
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build