`spack -V` stopped working when we added the `releases/latest` tag to
track the most recent release. It started just reporting the version,
even on a `develop` checkout. We need to tell it to *only* search for
tags that start with `v`, so that it will ignore `releases/latest`.
`spack -V` also would print out unwanted git eror output on a shallow
clone.
- [x] add `--match 'v*'` to `git describe` arguments
- [x] route error output to `os.devnull`
c05fa25057