fix: account for trailing newlines in commit messages - #495

Note this is a bit brittle - but getting this out & preventing countless infinite loops in automated CI seems like a higher priority. Will need to check that commit messages are stripped for comparison. Changing in get_commit_log seems to break tests for changelog generation, so better to strip it at the comparison site for now imo.

The bug causing #490 is that in git logs, commit messages have a trailing newline, which isn't accounted for in the message built up from config.

Have added some extra test cases to check for this.

@d-ryzhikov - hopefully this doesn't break the testing/use case you had for implementing your PR?
@zjgoodman - does this fix #490 for you?
Also as @danth reviewed the first PR - I guess it'd help if you could review this too please?

Going to merge this straight away so that we don't affect any more repositories with the infinite looping.