Don't show a trailing blank line at the end of single-column diffs

[StressTestor]

what

diffing a file that ends in a newline against /dev/null (a file addition or removal) shows an extra blank line after the last line:

$ difft /dev/null file.txt
file.txt --- Text
1 foo
2
3 bar
4        <- phantom, the file only has 3 lines

this is #966.

why

split_on_newlines returns a trailing empty element when the source ends in a newline (documented and intentional). the two-column display already drops that trailing element, but the single-column path (used when one side is empty, e.g. a file addition or removal) rendered it as a real line.

fix

pull the existing trailing-empty check into a small has_trailing_blank_line helper and apply it in the single-column branches too. the two-column path keeps the same behaviour (it reuses its existing line vec, no extra allocation). a genuine trailing blank line, e.g. a file ending in two newlines, is preserved; only the empty element from the terminating newline is dropped.

tests

• no_phantom_trailing_line in tests/cli.rs: diffs a 3-line file against /dev/null and asserts there is no line 4. fails before the fix, passes after.
• a unit test for the helper.
• regenerated the many_newlines entry in sample_files/compare.expected, which is a single-column (file addition) case whose output this corrects.

Fixes #966.