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 "-")
syntax: glob
*.elc
*.orig
*.rej
*~
*.o
*.so
*.pyc
*.swp
*.prof
tests/.coverage*
tests/annotated
tests/*.err
build
contrib/hgsh/hgsh
dist
doc/*.[0-9]
doc/*.[0-9].gendoc.txt
doc/*.[0-9].{x,ht}ml
MANIFEST
patches
mercurial/__version__.py
syntax: regexp
^\.pc/
Output/Mercurial-[0-9.]*.exe