sparse-checkout: fix OOM error with mixed patterns

Add a test to t1091-sparse-checkout-builtin.sh that would result in an
infinite loop and out-of-memory error before this change. The issue
relies on having non-cone-mode patterns while trying to modify the
patterns in cone-mode.

The fix is simple, allowing us to break from the loop when the input
path does not contain a slash, as the "dir" pattern we added does not.

This is only a fix to the critical out-of-memory error. A better
response to such a strange state will follow in a later change.

Reported-by: Calbabreaker <calbabreaker@gmail.com>
Helped-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/sparse-checkout.c

@@ -483,7 +483,7 @@ static void insert_recursive_pattern(struct pattern_list *pl, struct strbuf *pat
 		char *oldpattern = e->pattern;
 		size_t newlen;
 
-		if (slash == e->pattern)
+		if (!slash || slash == e->pattern)
 			break;
 
 		newlen = slash - e->pattern;

t/t1091-sparse-checkout-builtin.sh

@@ -103,6 +103,17 @@ test_expect_success 'clone --sparse' '
 	check_files clone a
 '
 
+test_expect_success 'switching to cone mode with non-cone mode patterns' '
+	git init bad-patterns &&
+	(
+		cd bad-patterns &&
+		git sparse-checkout init &&
+		git sparse-checkout add dir &&
+		git config core.sparseCheckoutCone true &&
+		git sparse-checkout add dir
+	)
+'
+
 test_expect_success 'interaction with clone --no-checkout (unborn index)' '
 	git clone --no-checkout "file://$(pwd)/repo" clone_no_checkout &&
 	git -C clone_no_checkout sparse-checkout init --cone &&