Problem #1219

Problem

.githooks/validate-k9.sh decides whether a nonconforming K9 file blocks a
commit with:

if [ -n "$CHANGED" ] && printf '%s\n' "$CHANGED" | grep -qxF "$file"; then

Under set -o pipefail, grep -q exits as soon as it matches and closes the
pipe. printf then receives SIGPIPE. On large change lists this produces false
negatives — a file that WAS touched is reported as untouched, so a change to a
nonconforming file slips through as advisory instead of blocking.

Measured at roughly 12–14% of runs on large change lists.

Fix

Use a here-string so there is no pipe and no SIGPIPE:

if [ -n "$CHANGED" ] && grep -qxF -- "$file" <<< "$CHANGED"; then

Acceptance

• A regression test that fails on the current form and passes on the fixed
  form: a large $CHANGED list where the file is present, asserted to be
  detected.
• The test lives under scripts/tests/ so scripts/run-shell-test-suite.sh
  picks it up.
• Fix and test land together.

Notes

Deliberately NOT bundled with the 9-archive district move (#1218), which only
added an is_fixture() case to this script.