Removed obsolete _git_tag attr
When doing test runs there may be no commits
Always ignore tags predating latest in changelog

If there are any missing releases in a changelog, the revision passed
to git log would be wrong, e.g from 0.001 to 0.006, instead of
one release to the next. This commit makes sure any tagged
versions smaller than the the latest changelog entry are skipped.

Also improves handling of tags which don't match version numbers.
Remove empty releases
v0.0.7

 - Always ignore tags predating latest in changelog [Ioan Rogers]
 - When doing test runs there may be no commits so add a placeholder entry
   [Ioan Rogers]