Merge branch 'jc/diff-no-index-in-subdir'

"git diff --no-index" run inside a subdirectory under control of a
Git repository operated at the top of the working tree and stripped
the prefix from the output, and oddballs like "-" (stdin) did not
work correctly because of it.  Correct the set-up by undoing what
the set-up sequence did to cwd and prefix.

* jc/diff-no-index-in-subdir:
  diff: --no-index should ignore the worktree

builtin/diff.c

@@ -487,6 +487,21 @@ int cmd_diff(int argc,
 
 	init_diff_ui_defaults();
 	repo_config(the_repository, git_diff_ui_config, NULL);
+
+	/*
+	 * If we are ignoring the fact that our current directory may
+	 * be part of a working tree controlled by a Git repository to
+	 * pretend to be a "better GNU diff", we should undo the
+	 * effect of the setup code that did a chdir() to the top of
+	 * the working tree.  Where we came from is recorded in the
+	 * prefix.
+	 */
+	if (no_index && prefix) {
+		if (chdir(prefix))
+			die(_("cannot come back to cwd"));
+		prefix = NULL;
+	}
+
 	prefix = precompose_argv_prefix(argc, argv, prefix);
 
 	repo_init_revisions(the_repository, &rev, prefix);

t/t4053-diff-no-index.sh

@@ -26,6 +26,23 @@ test_expect_success 'git diff --no-index directories' '
 	test_line_count = 14 cnt
 '
 
+test_expect_success 'git diff --no-index with -' '
+	cat >expect <<-\EOF &&
+	diff --git a/- b/-
+	new file mode 100644
+	--- /dev/null
+	+++ b/-
+	@@ -0,0 +1 @@
+	+frotz
+	EOF
+	(
+		cd a &&
+		echo frotz |
+		test_expect_code 1 git diff --no-index /dev/null - >../actual
+	) &&
+	test_cmp expect actual
+'
+
 test_expect_success 'git diff --no-index relative path outside repo' '
 	(
 		cd repo &&