Followup 5c0c117: make the edit tab work with oldid= again

After 5c0c117, clicking the VE edit tab on /wiki/Foo?oldid=123
sent you to /wiki/Foo?veaction=edit , dropping the oldid parameter
and editing the current version rather than the old version.

Fix this by only recomputing veEditUri from scratch when necessary,
and reusing the current URI when possible.

Change-Id: Iee9a745990f1a2bad40f46b8cdaaa727bb7fead8