If spack is checked out in a git worktree (see [1]), all git-related
commands fail because the `spack_is_git_repo()`-check is not thorough
enough.
When developing in a feature-branch in a seperate worktree, this is
annoying as all unittests regarding git-related spack commands fail,
cluttering the test results with false-positives.
[1]: https://git-scm.com/docs/git-worktree
Change-Id: I94b573a2c0e058e9ccc169e7ee6561626fbb06fd
1cff717ca8