[JENKINS-21952] Resolve tags with slashes

If no revisions are found, try to use the branch as explicit reference.
This could be a tag rel/my-tags or any other explicit reference like a
gerrit changeset (refs/changes/xx/yy/z).
Trivial improvement

removed unncessary addAll call.
Merge pull request #205 from pauxus/21952-tags-with-slashes

[JENKINS-21952] Resolve tags with slashes
Add test that a tag with a slash can be a branch spec