Handle all_changed_files=None in get_possible_conflicts
Compare upstream with common ancestor to see if a rebase might be needed
Add a test for list_upstream_changes
Use a custom PR representation so #NUMBER can be auto-linked
Unrelated cleanup, use get_current_sha1 in safe_merge.
Move conflict msg creation into a separate function

flake8 was complaining about an overly complex method:
scc/git.py:1467:1: C901 'GitRepository.merge_pull' is too complex (12)
Prefix upstream conflicts with 'Possible conflicts:'
list_merged_files relative to current merged PRs
merge master -Dorg -Ssuccess-only: PR 181 (Try to fix intermittent SSL error in RebaseTest.has_rebased_pr())
merge master -Dorg -Ssuccess-only: PR 186 (Comment on possible conflicts)
merge master -Dorg -Ssuccess-only

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

Merged PRs:
  # PR 181 sbesson 'Try to fix intermittent SSL error in RebaseTest.has_rebased_pr()'
  # PR 186 manics 'Comment on possible conflicts'

Generated by SCC-merge#406 (http://ci.openmicroscopy.org/job/SCC-merge/406/)