Fix substring bug

When one directory was a substring of another the directory would appear
as a subdirectory instead of a sibling

Fixed originally by this: 6e03dd77e6
Broken by this: db6c8a019d

Added integration test as this has bitten me before

src/test_dir2/dir/hello

@@ -0,0 +1 @@
+hello

src/test_dir2/dir_name_clash

@@ -0,0 +1 @@
+hello

src/test_dir2/dir_substring/hello

@@ -0,0 +1 @@
+hello