cmd/go: implement accurate pseudo-versions for Mercurial
This is CL 124515 (which only did Git) but for Mercurial,
quite a few years late. I'm not sure why we didn't do it
at the time - probably Mercurial blindness. Do it now.
Change-Id: I28f448a19143f7ce3de337cd1891bae86023b499
Reviewed-on: https://go-review.googlesource.com/c/go/+/718502 Reviewed-by: Michael Matloob <matloob@golang.org> Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>