fix(tree): std::path::Component has infallible conversion to &[u8]

[datdenkikniet]

The checks/conversions done in the _by_path methods in Tree and TreeRefIter are unnecessary, so this PR removes them in favor of using OsStr::as_encoded_bytes().

Since as_os_str().as_encoded_bytes() takes &self and (as far as I know) no platforms use internal mutability on OsStr, the underlying value must already have the form we want. The only thing that os_str_into_bstr does for us is validate that the byte string contains valid UTF-8. However, the only thing these BStrs are used for is comparison to other BStrs. Whether either of them is valid UTF-8 does not impact the function in any way (as string conversion errors are silently ignored), so we can simply skip the whole validation step and get ever-so-slightly-cheaper lookups by path.

Please let me know if this change is useful/welcome or not, I can imagine that there may be some case I have not thought of that breaks from this change.

[Sebastian Thiel (Byron)]

Thanks a lot, this is a great catch!

The previous implementation clearly worked around the fallibility of the conversion call, and the new version is faster with a similar failure mode: If this lookup is made on a platform with different-than-Git path encodings, and Git may try to turn paths to UTF-8, the equality comparison might not pan out.

But getting this right is hard as:

• it doesn't care about the case-sensivity settings of the repository
• it will happily compare different encodings and fail for that reason, with the only way to know how to do that by knowing which Git (for Windows) client was used to create the tree

So this seems to be an improvement, and for everything else let's wait and see.