Problem: Currently comment detection, addition, and removal are done
by matching 'commentstring' exactly. This has the downside when users
want to add comment markers with space (like with `-- %s`
commentstring) but also be able to uncomment lines that do not contain
space (like `--aaa`).
Solution: Use the following approach:
- Line is commented if it matches 'commentstring' with trimmed parts.
- Adding comment is 100% relying on 'commentstring' parts (as is now).
- Removing comment is first trying exact 'commentstring' parts with
fallback on trying its trimmed parts.
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