When creating the DeleteRef, the prefix should be provided.
Also update the tests to make sure it works. For DeleteBranches this
is done by amending the list of inputs to include a branch that does
not have the refs/heads prefix.
For DeleteBranch, which uses the API, we need to introduce another
test that explicitly uses the REST API. This means we can no longer
annotate it as @NoHttpd.
Bug: Issue 6591
Change-Id: I0d639d2cc261a51086a36a8fb1967b121b508f4f
bfea61824b