If a remote branch is referenced as refs/heads/<branch>, also drop the
heads/ from the local branch name to have a consistent naming.
This wasn't noticed so far because the primary use case of refs/ is
addressing symbolic refs in upstream, and the related test case
(test_refspec_absolute) was papering over another hidden kas issues
while using refs/heads/.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>