[release-branch.go1.8] cmd/go: correct directory used in checkNestedVCS test
This error was not used when using git because nested git is permitted.
Add test using Mercurial, so that at least we have a test, even though
the test is not run by default.
Fixes #22157
Fixes #22201
Change-Id: If521f3c09b0754e00e56fa3cd0364764a57a43ad
Reviewed-on: https://go-review.googlesource.com/69670
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Russ Cox <rsc@golang.org>