+git-tar-tree behaves differently when given a tree ID versus when given
+a commit ID or tag ID. In the first case the current time is used as
+modification time of each file in the archive. In the latter case the
+commit time as recorded in the referenced commit object is used instead.
+Additionally the commit ID is stored in a global extended pax header.
+It can be extracted using git-get-tar-commit-id.
+