Handle modifying rc files that don't have a trailing newline

[mythmon]

This changes the flow so that instead of checking if &rc.is_file, it just tries to read the file and handles the error. I think this an ok way to handle it, though it might produce a little more IO. It simplifies the logic a lot.

I couldn't find any tests that cover this function, and I'm not comfortable adding tests right now, but I could try and tackle that with some guidance, or if there is a similar test I could use an example.

Fixes #2636.

[mythmon]

I found the tests in the CI build, and added one for this. I ran cargo test before pushing this PR. Any idea why it didn't run that test?

[kinnison]

I'm not sure why the test didn't run for you locally, you can clearly see it running in CI here - https://github.com/rust-lang/rustup/pull/2667/checks?check_run_id=1914524215#step:17:1345

[kinnison]

One potential tweak to be done to coments in the tests, and a break which I think should be a continue

Otherwise this looks good, thank you for making the effort to contribute.

[kinnison]

This content is looking excellent, thank you for the extra test too, that is very useful.

Could you rebase the commits so that it's a clean change, and then we can look to merge this.

Thank you for your work.

[mythmon]

Alright, rebased and squashed down to one commit. Thanks for the review and guidance!

[kinnison]

👍

I'll merge on green-CI