sparse-checkout: detect short patterns

In cone mode, the shortest pattern the sparse-checkout command will
write into the sparse-checkout file is "/*". This is handled carefully
in add_pattern_to_hashsets(), so warn if any other pattern is this
short. This will assist future pattern checks by allowing us to assume
there are at least three characters in the pattern.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -651,7 +651,8 @@ static void add_pattern_to_hashsets(struct pattern_list *pl, struct path_pattern
 		return;
 	}
 
-	if (strstr(given->pattern, "**")) {
+	if (given->patternlen <= 2 ||
+	    strstr(given->pattern, "**")) {
 		/* Not a cone pattern. */
 		warning(_("unrecognized pattern: '%s'"), given->pattern);
 		goto clear_hashmaps;

t/t1091-sparse-checkout-builtin.sh

@@ -339,4 +339,13 @@ test_expect_success 'pattern-checks: /A/**/B/' '
 	check_files repo/deep/deeper1 "deepest"
 '
 
+test_expect_success 'pattern-checks: too short' '
+	cat >repo/.git/info/sparse-checkout <<-\EOF &&
+	/*
+	!/*/
+	/a
+	EOF
+	check_read_tree_errors repo "a" "disabling cone pattern matching"
+'
+
 test_done