[srccommit] Fix minor revision generation in presence of mixed tags
The 'authordate' key passed to git tag -l is only valid for lightweight
tags. Meanwhile, a '*authordate' key is only valid for annotated tags. As
such, neither is suitable for finding the latest tag from the combined set
of lightweight and annotated tags, which is a problem because the CVS-derived
tags are lightweight and the ones generated during GitLab merges are
annotated.
Fix this by using a git for-each-ref script that selects the appropriate
date from each tag before piping it through sort.