Demonstrate breakage: checkout overwrites untracked symlink with directory

This adds tests where an untracked file and an untracked symlink are in the
way where a directory should be created by 'git checkout'. Commit b1735b1a
(do not overwrite files in leading path, 2010-12-14) fixed the case where
a file is in the way, but the untracked symlink is still removed silently.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2021-checkout-overwrite.sh

@@ -0,0 +1,50 @@
+#!/bin/sh
+
+test_description='checkout must not overwrite an untracked objects'
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+
+	mkdir -p a/b/c &&
+	>a/b/c/d &&
+	git add -A &&
+	git commit -m base &&
+	git tag start
+'
+
+test_expect_success 'create a commit where dir a/b changed to file' '
+
+	git checkout -b file &&
+	rm -rf a/b &&
+	>a/b &&
+	git add -A &&
+	git commit -m "dir to file"
+'
+
+test_expect_success 'checkout commit with dir must not remove untracked a/b' '
+
+	git rm --cached a/b &&
+	git commit -m "un-track the file" &&
+	test_must_fail git checkout start &&
+	test -f a/b
+'
+
+test_expect_success SYMLINKS 'create a commit where dir a/b changed to symlink' '
+
+	rm -rf a/b &&	# cleanup if previous test failed
+	git checkout -f -b symlink start &&
+	rm -rf a/b &&
+	ln -s foo a/b &&
+	git add -A &&
+	git commit -m "dir to symlink"
+'
+
+test_expect_failure SYMLINKS 'checkout commit with dir must not remove untracked a/b' '
+
+	git rm --cached a/b &&
+	git commit -m "un-track the symlink" &&
+	test_must_fail git checkout start &&
+	test -h a/b
+'
+
+test_done