gerrit: add messageWithFooter to RevisionInfo
gerrit: add messageWithFooter to RevisionInfo

I am working on a change in chromium/infra
https://chromium-review.googlesource.com/c/infra/infra/+/1379425
We use the gerrit library's structs and currently it
doesn't include the messageWithFooter property in object.
So this change adds it.

Change-Id: I95cc8c25b2997d02548df462778b2b264c2db5cf
Reviewed-on: https://go-review.googlesource.com/c/156438
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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