x/build/cmd/gerritbot: does not respect codereview.cfg for fixing commit messages

https://go-review.googlesource.com/112277, which came from PR golang/vgo#4, said "Fixes #24099." The git-codereview tool would have read the vgo repo's codereview.cfg and automatically rewritten that line to say "Fixes golang/go#24099." This helps both authors and reviewers, neither of whom have to think about this anymore. The PR-bot should do the same, because as a reviewer I certainly don't want to lapse back into being in charge of catching this trivial but common mistake. Because the bot did not fix the commit message, the eventual submit did not close the issue.