cmd/coordinator: move git ancestor checking before buildlet creation, generalize

This also adds GoDeps to the builders config file and adds a
dependency for the ssacheck builder, which currently fails on older
release branches with:

> Failed on linux-amd64-ssacheck:
> https://storage.googleapis.com/go-build-log/1255edaf/linux-amd64-ssacheck_18f7e015.log
>
> ...
> compile: unknown debug key -d dclstack

Change-Id: I4ad0a298705feed56f467f3c8d9488f8be2e4af5
Reviewed-on: https://go-review.googlesource.com/43774
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed