Fix dirty calculation on cancellation of an inline editor.

cancel() was updating the dirty state in order to determine if it should
show a prompt. However, unlike save(), we don't want to unconditionally
do this, as we can end up in a state where the initial and current
fields don't match, but the user hasn't actually changed anything
(depending on the initial value set in an edited field).

We now call dirty() instead, which will properly handle forcing a
computation only if necessary.

Reviewed at https://reviews.reviewboard.org/r/6442/