Fix and refactor make_default_short_help (issue #1849)

[janluke]

Fixes #1849.

I found the old code a little bit difficult to follow, so I rewrote it. As it written now, the code can also be easily generalized to an arbitrary overflow placeholder.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [Not relevant] Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [Not relevant] Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.