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 "-")
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<title>#repo|escape#: #file|escape# annotate</title>
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<a href="?cl=#rev#">changelog</a>
<a href="?tags=">tags</a>
<a href="?cs=#node|short#">changeset</a>
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<a href="?f=#filenode|short#;file=#file|urlescape#">file</a>
<a href="?fl=#filenode|short#;file=#file|urlescape#">revisions</a>
<a href="?fa=#filenode|short#;file=#file|urlescape#;style=raw">raw</a>
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<h2>Annotate #file|escape#</h2>
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<td class="metatag">changeset #rev#:</td>
<td><a href="?cs=#node|short#">#node|short#</a></td></tr>
#rename%filerename#
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<td class="metatag">author:</td>
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<td class="metatag">date:</td>
<td>#date|date# (#date|age# ago)</td></tr>
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<td class="metatag">permissions:</td>
<td>#permissions|permissions#</td></tr>
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