Make on_a_tag function more accurate

Originally created by @intrigeri on #11490 (Redmine)

It currently answers “yes, sure” for any detached head, not just on a tag, which is OK for our current use case of this function, but if reused in another context it may break.

Also, this function should allow building a commit that has a tag on it: this new “building on a tag” requirement breaks builds on Jenkins while releasing, which is pretty bad since we’d love to have Jenkins’ input at this time.

Feature Branch: https://mraw.org/git/?p=tails.git;a=commit;h=beb3af631709fc6751929a869a98a23665db7148

Parent Task: sysadmin#5926 (closed)