version.py does not hard-code githash for releases

When making a release of Astropy (ex: a source distribution tarball) or even when just installing Astropy out of the git repo, although the version string gets hard-coded (along with the "revision count number"), the git hash itself does not get hard-coded when version.py is generated.  The git hash is only looked up by calling `git`.

Just as there is a `_last_generated_version` written to `version.py` there should also be a `_last_githash` or something.  `get_git_devstr` would try to determine the current git hash, but failing that it would fall back on the `_last_githash` (which for installed versions or for source dists would be the githash that was installed/released).