Fix #23646: relpath recognizes Windows drive letters

[belamenso]

Fixes #23646.
This pull introduces two changes in behavior of relpath. All changes and tests should apply only to platforms on which Sys.iswindows() is true.

1. For relpath(a,b) where a and b are from different Windows drives, abspath(a) is returned.
2. Paths are still case-sensitive, but not in the drive letter part preventing eg. relpath("C:\\a", "c:\\") == "..\\C:\\a".

[belamenso]

Can somebody review this?

[musm]

Thanks, this does improve the situation, but I think there are still issues we need to resolve with relpath, which might require bigger changes. See my comments in #23646

[musm]

In particular

relpath("d:x","d:a/b/c")  returns "..\\..\\..\\d:x" but should return  "..\\..\\..\\x"      ("d:a/b/c" is a relative path in the current directory in the d drive)

[belamenso]

The code should now correctly handle drive prefixes in relative paths, as verified by additional tests. I did not implement throwing ArgumentError for relpath between different drives, in such cases the absolute path of the requested destination is returned. Adding an exception to this method would be a breaking change.

relpath("d:x","d:a/b/c") == "..\\..\\..\\x"
relpath("c:/Users/vavasis/Documents", "E:/") == "c:\\Users\\vavasis\\Documents"
relpath("D","d") == "..\\D"
relpath("d:\\x", "D:\\x") == "."
relpath("c:/Users/vavasis/Documents", "C:/users/vavasis/Documents") == "..\\..\\..\\Users\\vavasis\\Documents"
relpath("c:/Users/vavasis/Documents", "c:/users/vavasis/Documents") == "..\\..\\..\\Users\\vavasis\\Documents"

@musm

[musm]

Can you add to the doc string about case insensitive of drive letters on Windows and also that if there's a drive mismatch we return the first path?

[belamenso]

Done.