DeleteChangeOp needs the ref prefix for the change that is being deleted
(e.g. refs/changes/12/34512/) so that it can find all refs of that
change by doing a ref search by prefix and then remove them.
The way how DeleteChangeOp computed the ref prefix was not very
readable. It retrieved the ref for the first patch set and then removed
the patch set number from the ref.
Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I42304dbcc0fa34b5ec9b12594f09151ee81d89d0
cb1647ac51