Code with a similar purpose was added in 568e0e5701 but got lost
when some things were moved from ve.Surface to ve.ce.Surface in
5012ed10.
Initializing the selection at (0,0) was known to cause problems before,
and since 789d0caf09 breaks editing of empty documents: typing in an
empty document begins in an inline slug, but SurfaceObserver doesn't
notice typing in an inline slug unless the ce.Surface pawns it, which
is OK because insertions in slugs are always pawned, but the pawning
logic believes the cursor to be at offset 0 where there is no slug
(it's at offset 1) and so it doesn't pawn.
Bonus: update tests and add descriptions for dm.Surface.change tests
Change-Id: Id72314d0fe650dacc7cdb842f5cea2f3bfba5145
d3af8b877b