Gerrit environment

This pull request includes 5 commits from pull request #51 (email for only a subset of refs) and 1 commit from pull request #52 (it's independent of the StashEnvironment, but if I didn't include that commit, you'd get annoying but trivial conflicts upon merging the two).  The relevant commits are the four at the end.

Gerrit doesn't use the standard git hooks; it employs entirely different ones of its own.  It also has some slightly different workflows for which we want to modify the defaults a bit.  Introduce a new GerritEnvironment and make git-multimail directly callable as a ref-update gerrit hook.