manifest: fix _manifest_content_at() for trees

We need to print the contents of the directory at manifest-rev, NOT
project.revision.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

src/west/manifest.py

@@ -1454,7 +1454,7 @@ def _manifest_content_at(project, path, rev=QUAL_MANIFEST_REV_BRANCH):
         # Importing a tree: return the content of the YAML files inside it.
         ret = []
         pathobj = PurePath(path)
-        for f in filter(_is_yml, project.listdir_at(path)):
+        for f in filter(_is_yml, project.listdir_at(path, rev=rev)):
             ret.append(project.read_at(str(pathobj / f),
                                        rev=rev).decode('utf-8'))
         return ret