differences in file lists are repeated at each depth level

This bug was originally reported by Mike Hommey (mh+reportbug@glandium.org) in Debian bug #989192:

Package: diffoscope
Version: 175
Severity: normal

STR:
- mkdir -p a/foo/bar b/foo/bar
- touch a/foo/bar/baz b/foo/bar/qux
- diffoscope a b --exclude-directory-metadata=recursive

Actual result:
--- a
+++ b
├── file list
│ @@ -1,3 +1,3 @@
│  foo
│  foo/bar
│ -foo/bar/baz
│ +foo/bar/qux
│   --- a/foo
├── +++ b/foo
│ ├── file list
│ │ @@ -1,2 +1,2 @@
│ │  bar
│ │ -bar/baz
│ │ +bar/qux
│ │   --- a/foo/bar
│ ├── +++ b/foo/bar
│ │ ├── file list
│ │ │ @@ -1 +1 @@
│ │ │ -baz
│ │ │ +qux

Expected Result:
--- a
+++ b
│   --- a/foo
├── +++ b/foo
│ │   --- a/foo/bar
│ ├── +++ b/foo/bar
│ │ ├── file list
│ │ │ @@ -1 +1 @@
│ │ │ -baz
│ │ │ +qux