Previously, we used `git describe --tags` to produce a version number
for non-released builds derived from the most recent tag + some info
such as the number of commits since that tag and then `g{HASH}`.
That always confuses people because the date portion at the front
looks old (it is typically the previous release) and the hash at
the end has that `g` in it.
This commit simplifies both the tag name used when making a release
and the computed version number take the date/time from the current
commit, and then append the hash. That way the version number always
corresponds to a commit.
This scheme doesn't help detect situations where the commit is
dirty, but I don't think the old one would have helped with that
either.
