effort: handling of $'s and \n's, plus make shellcheck happy

[nicolaiskogheim]

This fixes an issue that was reported this week, with $'s in a filename breaking git log and take git effort with it. git effort now understands how to treat files with actual newlines in them as well.

Shellcheck had some complaints, and I agreed to all of them. Because of that, there is a lot of code to review, so please look at each commit individually, if that makes it easier.

While I were at it, I expanded upon the docs, and made some small rewordings. If there are many comments on the doc edits, I'll split that out in it's own PR.

---

Please help test this

• [x] macOS
• [x] Linux
• [ ] BSD

You can use this repo as a test repo. It contains files with names that should produce an error if the issue isn't fixed.

[spacewander]

Sorry for my missing of this. Pass under Linux with bash 4.3.30.

@nicolaiskogheim Could we merge the pr now?

[nicolaiskogheim]

I'll fire up a FreeBSD box tomorrow and run the script just to be sure.