ref: Fix FindAllTags' pipeline code not returning nested tags

The new pipeline-based code in FindAllTags has been found to return less tags than the old implementation. As it turns out, the pipeline will not list any tags which are nested (e.g. "refs/tags/foo/bar"). The root cause is quite simple: we call git for-each-ref refs/tags/*, which will only list direct decendants of "refs/tags/" without going any deeper because of the trailing glob.

Fix the issue by properly listing the whole tags namespace by passing "rafs/tags/" instead, without the trailing glob.

Changelog: fixed