Comment on possible conflicts

Add a list of *possible* conflicting PRs and files as a comment on conflicting PRs by looking for files which are changed by multiple PRs (though this doesn't mean the PRs actually conflict). See for example https://github.com/ome/omero-pychrm/pull/35#issuecomment-51365579 (note that's since been rebased).

Todo: PR string formatting so that the PR number willl auto-link, is it safe to modify https://github.com/manics/snoopycrimecop/blob/56887c0c53f3fc0d2d5f6045254e39466de0d252/scc/git.py#L512 ?