Improve auto changelog script (#6753)

* Improve auto changelog script

The auto changelog script was creating empty or invalid entries in a
number of different cases, such as when the body of a commit spanned
multiple lines. This has been fixed, and the following additional
improvements have been made:

- Error handling (it will now crash upon encountering an error)
- Commits without a PR number in the subject are listed without the PR
prefix
- Invalid shellcheck warnings ignored
- Only the first line of the commit body is shown
- Carriage returns are stripped (some commits contain them)

This script should be more reliable for helping to manually update the
changelog. It's still not sufficiently robust to use as part of an
automated process - I don't think that's feasible without maintaining
stricter control over commit messages conventions and/or merge
strategies.

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