Do not push deletes for refs configured to be skipped

When pushing a delta set of refs, instead of treating non-existent and
!canPush() refs the same, use the same logic as when pushing all refs
and log and skip any !canPush() refs.

Release-Notes: refs configured to be skipped are not pushed as deletes
Change-Id: I3491c907580b16ea23493a5eaa325d2ef7d6d108