simplemerge: avoid quadratic concatenation when building output text
I haven't checked if the difference is measurable, but the new version
is no less readable or idiomatic, so I don't think performance numbers
are needed.
Differential Revision: https://phab.mercurial-scm.org/D9549
{header}
<title>Error</title>
<description>Error</description>
<item>
<title>Error</title>
<description>{error|escape}</description>
<guid>https://mercurial-scm.org/#error</guid>
</item>
</channel>
</rss>