diff sessions restore as simple splits

When opening vim with -d (diff) mode, two windows appear in a diff-fashion (e.g. marked lines etc.). When saving and opening such a session, the result is the same two windows, without the diff.

Reproduce:
1. create two files a.txt and b.txt
2. vim -d a.txt b.txt
3. :SaveSession diff (you can see the diffs marked)
4. :OpenSession diff (you cannot see the diffs marked)