tests: assert *.txt SYNOPSIS and -h output

Add a test to assert basic compliance with the CodingGuidelines in the
SYNOPSIS and builtin -h output. For now we only assert that the "-h"
output doesn't have "\t" characters, as a very basic syntax check.

Subsequent commits will expand on the checks here as various issues
are fixed, but let's first add the test scaffolding.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0450-txt-doc-vs-help.sh

@@ -0,0 +1,44 @@
+#!/bin/sh
+
+test_description='assert (unbuilt) Documentation/*.txt and -h output'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+test_expect_success 'setup: list of builtins' '
+	git --list-cmds=builtins >builtins
+'
+
+help_to_synopsis () {
+	builtin="$1" &&
+	out_dir="out/$builtin" &&
+	out="$out_dir/help.synopsis" &&
+	if test -f "$out"
+	then
+		echo "$out" &&
+		return 0
+	fi &&
+	mkdir -p "$out_dir" &&
+	test_expect_code 129 git $builtin -h >"$out.raw" 2>&1 &&
+	sed -n \
+		-e '1,/^$/ {
+			/^$/d;
+			s/^usage: //;
+			s/^ *or: //;
+			p;
+		}' <"$out.raw" >"$out" &&
+	echo "$out"
+}
+
+HT="	"
+
+while read builtin
+do
+	# -h output assertions
+	test_expect_success "$builtin -h output has no \t" '
+		h2s="$(help_to_synopsis "$builtin")" &&
+		! grep "$HT" "$h2s"
+	'
+done <builtins
+
+test_done