sparse-checkout: allow one-character directories in cone mode

In 9e6d3e64 (sparse-checkout: detect short patterns, 2020-01-24), a
condition on the minimum length of a cone-mode pattern was introduced.
However, this condition was off-by-one.

If we have a directory with a single character, say "b", then the
command

	git sparse-checkout set b

will correctly add the pattern "/b/" to the sparse-checkout file. When
this is interpeted in dir.c, the pattern is "/b" with the
PATTERN_FLAG_MUSTBEDIR flag. This string has length two, which satisfies
our inclusive inequality (<= 2).

The reason for this inequality is that we will start to read the pattern
string character-by-character using three char pointers: prev, cur,
next. In particular, next is set to the current pattern plus two. The
mistake was that next will still be a valid pointer when the pattern
length is two, since the string is null-terminated.

Make this inequality strict so these patterns work.

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

dir.c

@@ -682,7 +682,7 @@ static void add_pattern_to_hashsets(struct pattern_list *pl, struct path_pattern
 		return;
 	}
 
-	if (given->patternlen <= 2 ||
+	if (given->patternlen < 2 ||
 	    *given->pattern == '*' ||
 	    strstr(given->pattern, "**")) {
 		/* Not a cone pattern. */

t/t1091-sparse-checkout-builtin.sh

@@ -417,10 +417,20 @@ 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_expect_success 'pattern-checks: not too short' '
+	cat >repo/.git/info/sparse-checkout <<-\EOF &&
+	/*
+	!/*/
+	/b/
+	EOF
+	git -C repo read-tree -mu HEAD 2>err &&
+	test_must_be_empty err &&
+	check_files repo a
+'
 
 test_expect_success 'pattern-checks: trailing "*"' '
 	cat >repo/.git/info/sparse-checkout <<-\EOF &&