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 "-")
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
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abort: pretxncommit hook exited with status 1
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added 1 changesets with 1 changes to 1 files
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1 files, 2 changesets, 2 total revisions