rtk git diff --check emits no output at all, silently discarding every whitespace diagnostic

rtk git diff --check emits no output at all, silently discarding every whitespace diagnostic

Version: rtk 0.49.0
Platform: Linux x86_64 (musl build, installed via install.sh)

Summary

git diff --check is a diagnostic mode whose entire value is its stdout: it prints one line per whitespace error and exits 2. Under rtk, stdout is empty while exit 2 is preserved.

An agent or pre-commit script therefore sees a failure with no reason attached. Worse, a script that reads the output to decide what to fix concludes there is nothing to fix, while the non-zero exit blocks the commit.

Reproduction

T=$(mktemp -d); cd "$T"; git init -q .
printf 'line one\nline two\n' > f.txt
git add -A; git -c user.email=t@t -c user.name=t commit -qm init
printf 'line one\nline two   \nline three\t\n' > f.txt   # add trailing whitespace

git --no-pager diff --check; echo "raw exit=$?"
rtk git diff --check;        echo "rtk exit=$?"

Actual

-- raw --
f.txt:2: trailing whitespace.
+line two
f.txt:3: trailing whitespace.
+line three
raw exit=2
-- rtk --
rtk exit=2

Raw: 4 lines of diagnostics + exit 2. rtk: zero bytes + exit 2.

Expected

--check output passes through unfiltered. It is already minimal (one line per offending line plus the offending line), so there is nothing to compress, and every line is actionable.

Why this one matters more than general diff reshaping

This is not a formatting difference — it is total loss of the only signal the command produces. Related but distinct from #1918 (rtk git diff breaking patch/--name-only/exit-code consumers): that issue is about generating a usable patch, whereas --check produces no patch at all and is purely a diagnostic whose output is destroyed.

Suggested fix

Treat --check (and --exit-code, --quiet, --no-patch) as diagnostic/pass-through modes in the git diff filter: detect the flag and emit raw stdout. These modes are all cases where the output is either tiny or deliberately empty, so filtering has no upside.

Discovery context

Found while auditing ~14,700 real RTK_DISABLED=1 bypasses in local agent session history. git diff --check appeared as a case where an agent bypassed rtk after getting "(no output)" with exit 2 and being unable to explain the failure.

Start by locating the Rust git diff filter described in the issue and reproduce the command with trailing whitespace using the provided shell steps. Treat --check and the other listed diagnostic modes as the scope, and verify that raw stdout is preserved while the exit status remains unchanged.