t6020: new test with interleaved lexicographic ordering of directories

If a repository has two files:
    foo/bar/baz
    foo/bar-2/baz
then a simple lexicographic ordering of files and directories shows
    ...
    foo/bar
    foo/bar-2
    foo/bar/baz
    ...
and the appearance of foo/bar-2 between foo/bar and foo/bar/baz can trip
up some codepaths.  Add a test to catch such cases.

t6020 might be a slight misfit since this testcase does not test any
kind of file/directory conflict.  However, it is similar in spirit to
some tests (4-6) already in t6020 that check cases where a *file* sorted
between a directory and the files underneath that directory.  This
testcase differs in that now there is a *directory* that sorts in the
middle.

Although merge-recursive currently has no problems with this simple
testcase, I discovered that it's very possible to accidentally mess it
up.  Further, we have no other merge or cherry-pick or rebase testcases
in the entire testsuite that cover such a case, so I felt like it would
be a worthwhile addition to the testsuite.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6020-merge-df.sh

@@ -104,4 +104,47 @@ test_expect_success 'modify/delete + directory/file conflict; other way' '
 	test_path_is_file letters~HEAD
 '
 
+test_expect_success 'Simple merge in repo with interesting pathnames' '
+	# Simple lexicographic ordering of files and directories would be:
+	#     foo
+	#     foo/bar
+	#     foo/bar-2
+	#     foo/bar/baz
+	#     foo/bar-2/baz
+	# The fact that foo/bar-2 appears between foo/bar and foo/bar/baz
+	# can trip up some codepaths, and is the point of this test.
+	test_create_repo name-ordering &&
+	(
+		cd name-ordering &&
+
+		mkdir -p foo/bar &&
+		mkdir -p foo/bar-2 &&
+		>foo/bar/baz &&
+		>foo/bar-2/baz &&
+		git add . &&
+		git commit -m initial &&
+
+		git branch main &&
+		git branch other &&
+
+		git checkout other &&
+		echo other >foo/bar-2/baz &&
+		git add -u &&
+		git commit -m other &&
+
+		git checkout main &&
+		echo main >foo/bar/baz &&
+		git add -u &&
+		git commit -m main &&
+
+		git merge other &&
+		git ls-files -s >out &&
+		test_line_count = 2 out &&
+		git rev-parse :0:foo/bar/baz :0:foo/bar-2/baz >actual &&
+		git rev-parse HEAD~1:foo/bar/baz other:foo/bar-2/baz >expect &&
+		test_cmp expect actual
+	)
+
+'
+
 test_done