#68 Diff page does not have it's own template file

[Mikhail Vladimirov]

While the most of the pages are generated from two
templates (html.tmpl for common stuff and something
like browse.tmpl or editpage.tmpl for specific things),
diff page use only one template: html.tmpl. I think,
that it should use it's own inner temple (e.g. named
diff.tmpl).

[Reini Urban]

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Good idea.