add a pre-commit hook that checks that tests are not focussed

.githooks/README.md

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+# Git Hooks
+
+This directory contains useful Git hooks for working with quic-go.
+
+Install them by running
+```bash
+git config core.hooksPath .githooks
+```

.githooks/pre-commit

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+#!/bin/bash
+
+# Check that test files don't contain focussed test cases.
+errored=false
+for f in $(git diff --cached --name-only); do
+	if [[ $f != *_test.go ]]; then continue; fi
+	output=$(grep -n -e "FIt(" -e "FContext(" -e "FDescribe(" "$f")
+	if [ $? -eq 0 ]; then
+		echo "$f contains a focussed test:"
+		echo "$output"
+		echo ""
+		errored=true
+	fi
+done
+
+if [ "$errored" = true ]; then
+	exit 1
+fi