checkout: fix ambiguity check in subdir

The two functions in parse_branchname_arg(), verify_non_filename and
check_filename, need correct prefix in order to reconstruct the paths
and check for their existence. With NULL prefix, they just check paths
at top dir instead.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -985,7 +985,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 		int recover_with_dwim = dwim_new_local_branch_ok;
 
 		if (!has_dash_dash &&
-		    (check_filename(NULL, arg) || !no_wildcard(arg)))
+		    (check_filename(opts->prefix, arg) || !no_wildcard(arg)))
 			recover_with_dwim = 0;
 		/*
 		 * Accept "git checkout foo" and "git checkout foo --"
@@ -1046,7 +1046,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 		 * it would be extremely annoying.
 		 */
 		if (argc)
-			verify_non_filename(NULL, arg);
+			verify_non_filename(opts->prefix, arg);
 	} else {
 		argcount++;
 		argv++;

t/t2010-checkout-ambiguous.sh

@@ -41,6 +41,15 @@ test_expect_success 'check ambiguity' '
 	test_must_fail git checkout world all
 '
 
+test_expect_success 'check ambiguity in subdir' '
+	mkdir sub &&
+	# not ambiguous because sub/world does not exist
+	git -C sub checkout world ../all &&
+	echo hello >sub/world &&
+	# ambiguous because sub/world does exist
+	test_must_fail git -C sub checkout world ../all
+'
+
 test_expect_success 'disambiguate checking out from a tree-ish' '
 	echo bye > world &&
 	git checkout world -- world &&

t/t2024-checkout-dwim.sh

@@ -174,6 +174,18 @@ test_expect_success 'checkout of branch with a file having the same name fails'
 	test_branch master
 '
 
+test_expect_success 'checkout of branch with a file in subdir having the same name fails' '
+	git checkout -B master &&
+	test_might_fail git branch -D spam &&
+
+	>spam &&
+	mkdir sub &&
+	mv spam sub/spam &&
+	test_must_fail git -C sub checkout spam &&
+	test_must_fail git rev-parse --verify refs/heads/spam &&
+	test_branch master
+'
+
 test_expect_success 'checkout <branch> -- succeeds, even if a file with the same name exists' '
 	git checkout -B master &&
 	test_might_fail git branch -D spam &&