I hear Chromium contributors tell that Google's gclient / depottools is very good. We should investigate whether anything from that tool would be useful for us. Filing as MozillaBuild, but feel free to move this around.
I think mach fulfills this role quite nicely. A (single) Mercurial extension providing commands for common tasks would also be nice. But then you have to worry about satisfying the Git contingent. At which point you either provide separate solutions for each VCS client or you abstract the differences through something like mach. I think this bug is destined for lingering because there's no actionable item to it. I think a discussion on a mailing list or a brown bag would be more appropriate.