Also, some whitespace for presentation.

Useful for some history editing tasks.

This seems to change to main according to
https://lore.kernel.org/git/pull.762.git.1605221038.gitgitgadget@gmail.com/.
Pre-emptive change (since I don't know when I'll be affected by it).

Touch files and git add them.

By default just show what will be done with the git clean aliases.
Require passing -f to actually delete the files.

Git configuration.
- Don't ignore all files, but set status.showUntrackedFiles to no, add
to post-merge hook.