diff --no-index: refuse to compare stdin to a directory

When the user runs

    git diff --no-index file directory

we follow the behavior of POSIX diff and rewrite the arguments as

    git diff --no-index file directory/file

Doing that when "file" is "-" (which means "read from stdin") does not
make sense so we should error out if the user asks us to compare "-" to
a directory. This matches the behavior of GNU diff and diff on *BSD.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-no-index.c

@@ -218,11 +218,13 @@ static void fixup_paths(const char **path, struct strbuf *replacement)
 {
 	unsigned int isdir0, isdir1;
 
-	if (path[0] == file_from_standard_input ||
-	    path[1] == file_from_standard_input)
-		return;
-	isdir0 = is_directory(path[0]);
-	isdir1 = is_directory(path[1]);
+	isdir0 = path[0] != file_from_standard_input && is_directory(path[0]);
+	isdir1 = path[1] != file_from_standard_input && is_directory(path[1]);
+
+	if ((path[0] == file_from_standard_input && isdir1) ||
+	    (isdir0 && path[1] == file_from_standard_input))
+		die(_("cannot compare stdin to a directory"));
+
 	if (isdir0 == isdir1)
 		return;
 	if (isdir0) {

t/t4053-diff-no-index.sh

@@ -205,4 +205,9 @@ test_expect_success POSIXPERM,SYMLINKS 'diff --no-index normalizes: mode not lik
 	test_cmp expected actual
 '
 
+test_expect_success 'diff --no-index refuses to diff stdin and a directory' '
+	test_must_fail git diff --no-index -- - a </dev/null 2>err &&
+	grep "fatal: cannot compare stdin to a directory" err
+'
+
 test_done