cmd/go: stamp VCS commit time into binaries

cmd/go: stamp VCS commit time into binaries

Only Git and Mercurial are supported for now.

This CL also:
- Skips tagging "revision" and "committime" for empty repositories.
- Stores the full Mercurial changeset ID instead of the short form.

Fixes #37475

Change-Id: I62ab7a986d1ddb2a0e7166a6404b5aa80c2ee387
Reviewed-on: https://go-review.googlesource.com/c/go/+/356251
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>