Improve messages about joining C: on the right

Although the path `C:` is relative because it indicates the current
directory on the C: drive, it still "takes over" when concatenated
after at most if not all paths (including absolute paths).

When the paths it is concatenated with are absolute paths with
drive letters other than C:, this behavior, while not obvious, is
conceptually clear such that no other behavior could be correct.
This is because the (drive-specific) current directory on the C:
drive is not located on another drive, and therefore not found
under any path that specifies a drive that is not C:.

(There are other situations where `C:` on the right takes
precedence where the explanation is less elegant, but they are
mostly not relevant to the assertions currently present here.)

This relates to:

- https://github.com/Byron/gitoxide/pull/1374/files/e955770c0b5c06a6c8518a06df4aa0cc3b506f16#r1721055278
- https://github.com/Byron/gitoxide/pull/1374/files/e955770c0b5c06a6c8518a06df4aa0cc3b506f16#r1721079186

Author: EliahKagan

gix-fs/tests/stack/mod.rs

@@ -92,7 +92,7 @@ fn path_join_handling() {
     assert_eq!(
         p("d:/").join("C:"),
         p("C:"),
-        "d-drive + c-drive = c-drive - interesting, as C: is supposed to be relative"
+        "d-drive + c-drive-relative = c-drive-relative - C: is relative but not on D:"
     );
     assert_eq!(
         p("d:\\").join("C:\\"),
@@ -113,7 +113,7 @@ fn path_join_handling() {
     assert_eq!(
         p("\\\\.").join("C:"),
         p("C:"),
-        "device-namespace-unc + win-drive-relative = win-drive-relative - c: was supposed to be relative, but it's not acting like it."
+        "device-namespace-unc + win-drive-relative = win-drive-relative - C: as a relative path is not the C: device, so this is not \\\\.\\C:"
     );
     assert_eq!(p("relative").join("C:"), p("C:"), "relative + win-drive = win-drive");