Fix lots of gen_release_notes bugs

Currently, there's no test code for the heuristic that attempts to extract Closes: lines from the body of all the commits in the release. Of course, that means it's broken. In a lot of different ways, it turns out.

First it doesn't correctly handle https://not.mesa correctly, treating those as issues for mesa. We need to be sure that we only bugs for mesa, and not another project are being counted

Second, it doesn't handle two issues on the same line, such as Closes: #1, #2.

Finally, it doesn't handle multiple Closes: directives, ignoring all but the last one.

This series addresses all of those issues, and adds tests to ensure that things continue to work.