rm: fix issue #6620 (refuse to remove '.' and '..')

[just-an-engineer]

Before processing the directory, if it ends with "." or "..", it now immediately returns with the same error message as GNU
Should close #6620

[just-an-engineer]

I see this does the same thing as #6621

[sylvestre]

but tests/rm/r-4 is fixed in the other PR
(your modification is smaller than the other which is a +)

[just-an-engineer]

Thanks. Do you want me to close this out? Or which PR will get merged?

[sylvestre]

the other PR code is a bit more complex.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/timeout/timeout is no longer failing!

[sylvestre]

@just-an-engineer what is your plan wrt this PR?

[just-an-engineer]

We can merge this one if @AnirbanHalder654322 doesn't have any progress on the other PR, since I already have it working without regex, it already works on Windows, and since I made them a coauthor on the commits. But if they responds and pushes whatever changes, I'll leave it to you or them, but if the other one gets merged, I can just close this one.
So I can go either way. It can be merged right now, or it can be closed if Anirban pushes the necessary changes and you decide to go with that one

[just-an-engineer]

@sylvestre, I'm marking this as a draft, and I'll close it out once @AnirbanHalder654322 PR gets merged. His is starting to look a bit cleaner and more fleshed out than mine. Particularly with removing trailing slashes, and more comprehensive tests, in my opinion.

[BenWiederhake]

1. The conversion is done twice. That's not a big deal, but maybe just saving the result in a variable is easy enough to avoid doing the same work twice?
2. .to_str().unwrap() will cause problems with non-UTF-8 filenames. Please consider using os_str_as_bytes: If it works great (and add a test), if it doesn't (or is too much work or feels out of scope) then disregard this suggestion.
3. I'm not sure whether this is the right condition. Maybe a trailing path separator ("del") should count, too? Here's a small example that demonstrates the difference:

   $ rm -rf foo && mkdir -p foo/bar/baz && rm -rf foo/bar/baz/../
   rm: refusing to remove '.' or '..' directory: skipping 'foo/bar/baz/../'
   [$? = 1]
   $ find foo/
   foo/
   foo/bar
   foo/bar/baz
   $ rm -rf foo && mkdir -p foo/bar/baz && cargo run -q --features rm rm -rf foo/bar/baz/../
   rm: cannot remove 'foo/bar/baz/../': No such file or directory (os error 2)
   [$? = 1]
   $ find foo
   foo
   foo/bar
   

   As you can see, we still delete a directory, causing a confusing error message. I'm not sure whether it suffices to add the two conditions ends_with("/./") and ".ends_with("/../")". Can you find any other edge cases?