pullrequests: fix "additional changes" js error - make sure all cs tables have a first column to align with (Issue #274)

A slight simplification of 16234f629cfb and fixing a JavaScript failure for
unauthenticated visits to PRs with pending available changesets.

nextFirstincell would be undefined becuase the "Current revision - no change"
line didn't have any elements in td, and nextFirstincell.offsetTop would thus
fail.

Fixed by adding a span around the text.

Also clarify that any element is fine - there is no need to check for
visibility.