fix(diff): preserve POSIX exit codes

[cancaries]

Fixes #2446.

Summary

• Preserve rtk diff compact output while returning POSIX-compatible exit codes.
• Return 0 when compared files are identical.
• Return 1 when compared files differ.
• Return 2 when either input file cannot be read.
• Add focused regression tests for all three exit-code paths.

Verification

• git diff --check

I could not run cargo test diff_cmd locally because this Windows environment does not have a Rust toolchain available.

[CLAassistant]

All committers have signed the CLA.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR updates the diff command to return meaningful process-style exit codes (0/1/2) instead of always returning success, and threads those exit codes back through the CLI dispatcher.

Changes:

• Change diff_cmd::run and diff_cmd::run_stdin to return Result<i32> exit codes (0 identical, 1 different, 2 trouble).
• Update CLI Commands::Diff handling to return the diff command’s exit code rather than always 0.
• Add unit tests validating the new exit-code behavior for identical, different, and missing-file scenarios.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/main.rs	Returns the diff subcommand’s exit code from the CLI match arm.
src/cmds/git/diff_cmd.rs	Implements exit-code semantics for diff operations and adds tests for the new behavior.

Comments suppressed due to low confidence (1)

src/cmds/git/diff_cmd.rs:1

• The exit-code values (0 and 1 here, plus 2 elsewhere) are currently “magic numbers”. Consider introducing named constants (e.g., EXIT_OK, EXIT_DIFF, EXIT_TROUBLE) or a small enum to centralize and self-document the exit-code mapping—this also reduces the chance of future inconsistencies between call sites and tests.

//! Compares two files and shows only the changed lines.

[KuSh]

Closing as a duplicate of #2473, which makes the same fix: a read failure on either operand returns POSIX diff's exit 2 instead of collapsing into exit 1.

Confirmed still live on 0.46.0 — rtk diff x /nope; echo $? prints 1. The other half of #2446 (differ → exit 1) is already fixed; this is what's left.

#2473 was picked because its tests exercise the read_operands helper directly. The three tests here call run(), which goes through timer.track(...) and writes into the machine's real tracking database — the problem #3758 is currently fixing. Your test_run_returns_zero_for_identical_files / ..._one_for_different_files pair is good coverage of the full contract, though; it's the kind of thing that belongs in tests/guard_integration_test.rs as a real-process test, which nothing currently has. If you'd like to submit that separately it would fill a genuine gap.