maintner: initial gerrit support

Pass --watch-gerrit on the command line to watch a Gerrit project. We
initialize a Git repository in $HOME/var/maintnerd/<gerrit-url>, then
fetch all metadata related to that project, and store it in the
Corpus.

Change-Id: I47a8b5e5af2ae1b99c97cc756790c8e21465e8ee
Reviewed-on: https://go-review.googlesource.com/38421
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
9 files changed