Check that merge --info runs for PRs and branches

Note this doesn't check the info is correct, only that it runs without an error.
Fix iteration over self.candidate_branches
Check private status of fork instead of origin
merge master -Dorg -Ssuccess-only: PR 186 (Comment on possible conflicts)
merge master -Dorg -Ssuccess-only: PR 193 (Use the PR repo)
merge master -Dorg -Ssuccess-only

Repository: openmicroscopy/snoopycrimecop
Already up-to-date.

Merged PRs:
  # PR 186 manics 'Comment on possible conflicts'
  # PR 193 manics 'Use the PR repo'

Generated by SCC-merge#478 (http://ci.openmicroscopy.org/job/SCC-merge/478/)