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 "-")
adding a
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% expect ssl error
pushing to http://localhost:20059/
searching for changes
ssl required
% expect authorization error
pushing to http://localhost:20059/
searching for changes
push not authorized
% expect authorization error: must have authorized user
pushing to http://localhost:20059/
searching for changes
push not authorized
% expect success
pushing to http://localhost:20059/
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
rolling back last transaction
% expect authorization error: all users denied
pushing to http://localhost:20059/
searching for changes
push not authorized
% expect authorization error: some users denied, users must be authenticated
pushing to http://localhost:20059/
searching for changes
push not authorized