revset: cache smartset's min/max
As the content of a smartset never changes, min and max will never change
either. This will save us time when this function is called multiple times.
This is relevant for
issue4782 but does not fix it.
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<!-- Error -->
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<link rel="self" href="{urlbase}{url|urlescape}atom-log"/>
<link rel="alternate" href="{urlbase}{url|urlescape}"/>
<title>Error</title>
<updated>1970-01-01T00:00:00+00:00</updated>
<entry>
<title>Error</title>
<id>http://mercurial.selenic.com/#error</id>
<author>
<name>mercurial</name>
</author>
<updated>1970-01-01T00:00:00+00:00</updated>
<content type="text">{error|escape}</content>
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</feed>