Some variants of Mercurial respond differently to “permission denied” errors
than to “file not found”, and we set HOME to point to an absolute path that may
produce the former instead of the latter.

To discourage Mercurial from trying HOME, give it an explicit (empty)
configuration in the working directory instead.

Change-Id: I82ae99a6892bba7fc3d41b77209ca181d24315e2
Reviewed-on: https://go-review.googlesource.com/136135Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>