Simply checking the exit code of `git rev-parse --git-dir` should
suffice here, but that requires deviating from the infrastructure
provided by `run`, so I've left that for a future change.

Originally by Tamir Duberstein but updated by iant & rsc to add
the filepath.Join logic.

Fixes #11211 (again).

Change-Id: I6d29b5ae39ba456088ae1fb5d41014cb91c86897
Reviewed-on: https://go-review.googlesource.com/18323Reviewed-by: Ian Lance Taylor <iant@golang.org>