commands.diff: add support for diffs relative to a subdirectory
Previous patches added all the backend support for this. This exposes this
option in the UI.
Ancestors of 5
4 2 0
Ancestors of 6 and 5
3 4 2 1 0
Ancestors of 5 and 4
4 2 0
Ancestors of 7, stop at 6
6
Ancestors of 7, including revs
7 6 5 3 4 2 1 0
Ancestors of 7, 5 and 3, including revs
7 5 3 6 4 2 1 0
Descendants of 5
7 8
Descendants of 5 and 3
6 7 8
Descendants of 5 and 4
5 7 8