pullrequests: don't show empty "additional changesets" (issue #280)

When opening a pullrequest on a revision range including the tipmost
revision, and then pushing a new revision on top of that, the PR page shows:
This pullrequest can be updated with changes on ...:
and then nothing.

This is because the available revisions looped over are in 'avail_cs' but
the guard around the loop is checking on 'avail_revs'. The former, while
derived from avail_revs, can become empty under circumstances like this one.

Fix the problem by changing the guard checking avail_cs rather than
avail_revs, and making sure the printed message is aligned to 'No additional
changes found'.