tests/test-hup.out

run-tests.py: fix diff output when test-foo.out doesn't exist.

Previously, if test-foo.out doesn't exist and test-foo outputs
something, the first line of the diff will look like this:

-+changeset:   0:0acdaf898367

(note the leading "-")

0
0
adding changesets
killed!
transaction abort!
rollback completed
00changelog.i
data
journal.dirstate