Fix /mergeable test to include accepted commits

Merge tests were failing due to a bug introduced into the Mergeable
implementation by 3a2a2471b581c454a6a157034d3d0039f2c13cde.

Merge strategies must be passed the current set of branch heads,
otherwise it appears as though the parents are not ready to submit
and are still pending code review.

Change-Id: I888f6507f8c68d114539d68314bcc4a38a36fc31
1 file changed