Use FQRN to prevent errors when branch = label

[hiddeco]

Fixes #1845

If the label and branch were set to the same name, Flux would not
create the sync tag as the revision lookup for HEAD and <tag>
returned the same revision.

This is due to how git looks up symbolic ref names, giving priority
to heads over tags. Using the (almost) fully qualified ref name will
tell git to only look for matches of that type.

https://git-scm.com/docs/gitrevisions#Documentation/gitrevisions.txt-emltrefnamegtemegemmasterememheadsmasterememrefsheadsmasterem

[squaremo]

Looks OK to me. SyncRevision explicitly checks a tag; HeadRevision is only used to get the head of a branch, so although the meaning of the latter is slightly changed (as noted in a comment), I think it's safe to use the unambiguous names.