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 "-")
include hg
recursive-include mercurial *.py
include hgweb.cgi hgwebdir.cgi
include hgeditor rewrite-log
include tests/README tests/coverage.py tests/run-tests.py tests/md5sum.py tests/test-*[a-z0-9] tests/*.out
prune tests/*.err
include *.txt
include templates/map templates/map-*[a-z0-9]
include templates/*.tmpl
include templates/static/*
include doc/README doc/Makefile doc/gendoc.py doc/*.txt doc/*.html doc/*.[0-9]
recursive-include contrib *
include README
include CONTRIBUTORS
include COPYING
include Makefile
include MANIFEST.in