ui-diff: fix links from diffstat

The links in the diffstat is supposed to work as a filter for the diff,
but this only worked when a single rev was supplied, i.e. the filtered
diff was always against the parent of the specified rev.

With this patch it is now possible to use the diffstat as a 'filter menu'
for urls like http://hjemli.net/git/cgit/diff/?id=v0.7.2&id2=v0.7.1

Signed-off-by: Lars Hjemli <hjemli@gmail.com>