`spack -V` is now more descriptive for dev branches

`spack -V` previously always returned the version of spack from
`spack.spack_version`.  This gives us a general idea of what version
users are on, but if they're on `develop` or on some branch, we have to
ask more questions.

This PR makes `spack -V` check whether this instance of Spack is a git
repository, and if it is, it appends useful information from `git
describe --tags` to the version.  Specifically, it adds:

  - number of commits since the last release tag
  - abbreviated (but unique) commit hash

So, if you're on `develop` you might get something like this:

    $ spack -V
    0.13.3-912-3519a1762

This means you're on commit 3519a1762, which is 912 commits ahead of
the 0.13.3 release.

If you are on a release branch, or if you are using a tarball of Spack,
you'll get the usual `spack.spack_version`:

    $ spack -V
    0.13.3

This should help when asking users what version they are on, since a lot
of people use the `develop` branch.

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