Use a consistent file path label when paths share no suffix

[StressTestor]

what

When diffing two files whose paths share no common trailing component, the path shown in the hunk header depends on whether the files have an extension:

$ difft a b
a --- Text
$ difft a.txt b.txt
b.txt --- Text

a vs b labels with the left path, but a.txt vs b.txt labels with the right path. This is #925.

why

build_display_path's no-common-suffix arm picked the rhs path when rhs had an extension and the lhs path otherwise:

None => {
    if rhs.extension().is_some() {
        rhs.display().to_string()
    } else {
        lhs.display().to_string()
    }
}

so the side shown was driven by the extension.

fix

Use the rhs path consistently here, matching the git-tmpfile branch just above (which already returns rhs) and the usual convention of showing the new/destination file.

None => rhs.display().to_string(),

The change is scoped to that one match arm. The reporter also suggested detecting a rename and showing both paths; that is a larger behaviour change, so I kept this minimal and easy to extend if you would rather take the show-both approach.

tests

test_build_display_path_consistent_without_common_suffix covers the extensionless and extensioned cases (both now use rhs) and the shared-suffix case (unchanged). It fails before the fix and passes after. Full suite green.

Fixes #925.