fix(git): pass through diff diagnostic modes like --check unfiltered (#3984)

[anujsingh-cse]

Closes #3984

Problem

rtk git diff --check emits no output at all — it silently discards every whitespace diagnostic. git diff --check is a diagnostic mode whose entire value is its stdout (one line per whitespace error, exit code 2). Under rtk, stdout was empty while exit 2 was preserved, so an agent or pre-commit script sees a failure with no reason attached.

Repro:

git --no-pager diff --check   # 4 lines of diagnostics, exit 2
rtk git diff --check          # 0 bytes, exit 2

Fix

Treat --check (plus the sibling diagnostic modes --exit-code, --quiet, --no-patch) as pass-through modes in the git diff filter. A wants_diagnostic_output helper detects these flags, and run_diff routes them to a raw pass-through branch that emits stdout and stderr verbatim and preserves the exit code — no --stat rewrite and no compaction.

Tests

• test_wants_diagnostic_output_detects_diff_modes — all four flags detected
• test_wants_diagnostic_output_ignores_normal_diff_args — --stat, --numstat, a path, and empty args are not diagnostic

cargo fmt --check and cargo clippy --all-targets clean; 478 git tests pass. Verified end-to-end: rtk git diff --check now prints the trailing-whitespace diagnostics and preserves exit code 2.

[rtk-wshm-sync-bot]

wshm · Automated triage by AI

📊 Automated PR Analysis

🐛 Type	bug-fix
🟢 Risk	low

Summary

Fixes rtk git diff --check (and sibling diagnostic flags --exit-code, --quiet, --no-patch) so they pass through stdout/stderr and exit code unfiltered instead of being silently discarded by rtk's diff compaction logic. Adds a wants_diagnostic_output helper and a raw pass-through branch in run_diff, with unit tests covering detection of the flags.

Review Checklist

• Tests present
• Breaking change
• Docs updated

Linked issues: #3984

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Analyzed automatically by wshm · This is an automated analysis, not a human review.

[KuSh]

Thanks for the fix and the repro. The same behaviour landed on develop through #3681 (merged 2026-09-12, commit 7931993): --check, --exit-code and --quiet are classified by the tokenizer as raw-shape flags and go straight through with git's stdout, stderr and exit code, and -s/--no-patch skip compaction since there is no body to compact. Verified on current develop with git 2.53: rtk git diff --check prints the whitespace report and exits 2, and the combined forms match raw git byte for byte. Closing as already fixed; if a diagnostic mode still comes back empty for you on develop, please comment here and it can be reopened.