Another approach would be possible with the new git tag options --merged (in git 2.7+, Q4 2015)
git tag --merged <abranchname>
See commit 5242860, ... (10 Sept 2015) by Karthik Nayak (KarthikNayak).
(Merged by Junio C Hamano -- gitster -- in commit 8a54523, 05 Oct 2015)
tag.c: implement '--merged' and '--no-merged' options
Use 'ref-filter' APIs to implement the '--merged' and '--no-merged' options into 'tag.c'.
- The '
--merged' option lets the user to only list tags merged into the named commit.
- The '
--no-merged' option lets the user to only list tags not merged into the named commit.
If no object is provided it assumes HEAD as the object.