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/manics/snoopys-sandbox/pulls (`scc merge -v dev_4_4 --comment`)
- manics/snoopys-sandbox#2
- manics/snoopys-sandbox#3
- manics/snoopys-sandbox#4

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 ?
Any reason not to combine both loops here? Since PR merging happens in the same order as the list, filing the `changes_files` dictionary as it goes should also be sensible for reporting conflicts (i.e. would prune out non yet merged PRs modifying the conflicting file to be listed)