When creating a tarball from the HEAD instead of a release tag, use the date to generate the filename instead of the Expat version number.  This makes sense since the resulting tarball does not represent a versioned release.
When creating a tarball from the HEAD instead of a release tag, use
the date to generate the filename instead of the Expat version
number.  This makes sense since the resulting tarball does not
represent a versioned release.

cgwalters/expat-git-mirror,cgwalters/expat-git-mirror,cgwalters/expat-git-mirror