tg-revert.sh: try to tame rev-parse --symbolic-full-ref

tg-revert.sh: try to tame rev-parse --symbolic-full-ref

Before giving up on a `git rev-parse --symbolic-ref-name` result,
explicitly prefix the given ref with "refs/tags/" and try once more
before deciding it's not usable.

If there are multiple matches causing --symbolic-ref-name to fail
(the list from `git help revisions`), we're unlikely to care about
anything not under refs/tags/... (except for refs/tgstash and that
can always be written --stash to disambiguate).

We're going to default to interpreting it as a refs/tags/...
something anyway (and then fail using that if it's invalid) so it
does no harm and it will provide an improved DWIM experience when
there happen to be branches and tags with the same name present.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>