cmd/go: make vcstest repo scripts compatible with old Mercurial versions

cmd/go: make vcstest repo scripts compatible with old Mercurial versions

The scripts added in CL 421455 passed on the TryBots, but failed on
the "-stretch" builders, which supply Mercurial 4.0
(released 2016-11-01).

Debian 9 “Stretch” has been at end-of-life since June 30, 2022, but
until we can turn down the outdated builders (#56414) we should keep
them passing tests.

For #27494.
Updates #56414.

Change-Id: I9df0ed452dfbfaeb1b4c0d869d02dd9ed21b3ff6
Reviewed-on: https://go-review.googlesource.com/c/go/+/445356
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>