t4054: test diff --no-index with stdin

"git diff --no-index" supports reading from stdin with the path "-".
There is no test coverage for this so add a regression test before
changing the code in the next commit.

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

t/t4053-diff-no-index.sh

@@ -205,6 +205,25 @@ test_expect_success POSIXPERM,SYMLINKS 'diff --no-index normalizes: mode not lik
 	test_cmp expected actual
 '
 
+test_expect_success "diff --no-index treats '-' as stdin" '
+	cat >expect <<-EOF &&
+	diff --git a/- b/a/1
+	index $ZERO_OID..$(git hash-object --stdin <a/1) 100644
+	--- a/-
+	+++ b/a/1
+	@@ -1 +1 @@
+	-x
+	+1
+	EOF
+
+	test_write_lines x | test_expect_code 1 \
+		git -c core.abbrev=no diff --no-index -- - a/1 >actual &&
+	test_cmp expect actual &&
+
+	test_write_lines 1 | git diff --no-index -- a/1 - >actual &&
+	test_must_be_empty 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