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 "-")
#!/bin/sh
hg init a
echo a > a/a
hg --cwd a ci -Ama
hg clone a c
hg clone a b
echo b >> b/a
hg --cwd b ci -mb
echo % push should push to default when default-push not set
hg --cwd b push | sed 's/pushing to.*/pushing/'
echo % push should push to default-push when set
echo 'default-push = ../c' >> b/.hg/hgrc
hg --cwd b push | sed 's/pushing to.*/pushing/'