fix(display): report one-sided empty files as changes

[YuriNachos]

Summary

difft foo bar where bar/bar.txt is an empty file that exists only in bar/ printed bar.txt --- Text / No changes., and --skip-unchanged silently dropped it. A file present in only one diffed directory is a change, so it is now reported as one. Empty files present on both sides are still reported as unchanged.

Addresses #1027.

Root cause

Directory diffing pairs files by relative path (relative_paths_in_either, src/files.rs) and compares each pair via diff_file with missing_as_empty = true (src/main.rs, diff_directories). For a file that exists on only one side, read_files_or_die (src/files.rs:24) substitutes empty content for the missing side — the existing behaviour needed for mercurial removals. For an empty file on the present side, both sides then compare equal, so diff_file_content took the early "identical sources" return (has_syntactic_changes: false), printing No changes. and hiding the file under --skip-unchanged. A one-sided non-empty file never hit this because its content differs from the empty substitute.

Changes

• src/files.rs: read_files_or_die now also returns a MissingFile { Neither, Lhs, Rhs } indicating which side (if any) was missing and substituted as empty. Single-file mode (missing_as_empty = false) behaviour is unchanged: an explicitly missing argument still errors with exit code 2.
• src/main.rs (diff_file):
  • Text path: when a side was missing, has_syntactic_changes is forced on. A one-sided file is a change even when its content produces no detectable differences — an empty file, content that normalizes to empty under the default --strip-cr=on, or whitespace-only content whose syntax tree matches the empty side's. Output is the existing header + Has changes. / Has syntactic changes.; --skip-unchanged keeps the file and --exit-code reports it.
  • Binary-overridden path: has_byte_changes is only None when the bytes are equal and nothing was missing, so a one-sided empty binary file reports Binary file added (0 B). rather than No changes.
• src/display/json.rs: with empty hunks, JSON status is now changed when has_syntactic_changes is set (previously always unchanged), keeping the JSON payload consistent with the text display and the --exit-code result. This also corrects --check-only --display=json, which previously reported changed files as unchanged.

Tests

8 new integration tests in tests/cli.rs using new fixtures under sample_files/empty_files_1/ and sample_files/empty_files_2/, all verified red on the pre-fix code and green after:

• one-sided empty file on the left → Has changes. and exit code 1 with --exit-code
• one-sided empty file on the right → Has changes.
• empty file on both sides → still No changes. (regression guard, anchored to the exact header + message)
• --skip-unchanged shows the one-sided empty file, hides the unchanged ones
• one-sided CR-only file (content normalizes to empty under --strip-cr=on) → Has changes.
• one-sided whitespace-only .py (syntax tree identical to the empty side's) → Has syntactic changes.
• --override-binary one-sided empty file → Binary file added (0 B).
• JSON output reports the one-sided empty file as "status":"changed"

cargo fmt --check, cargo clippy --all -- -D warnings and cargo test (122 unit + 31 CLI) all pass.

Known limitations, deliberately out of scope here: git's /dev/null form of one-sided files (used by git difftool for added/deleted files) is indistinguishable from an explicit user /dev/null comparison and still reports No changes.; a one-sided empty binary file's added/removed wording keys off byte lengths (the pre-existing TODO: Fix this pedantic case in print_diff_result); JSON uses changed rather than created/deleted for one-sided empty files because the missing side is not currently recorded in DiffResult.

Disclosure

This change was developed with AI assistance (Claude) and human-reviewed; I have read all the code in the diff and can discuss every line.