Fixes to "hg heads -r FOO": Make it actually work (undefined variable 'rev'; allow to pass a rev parameter). repo.branchlookup() doesn't need a copy of heads because it doesn't modify it. Use None as default argument to heads() instead of nullid. Doc string PEPification.

#header#
    <title>#repo|escape# Changelog</title>
    <description>#repo|escape# Changelog</description>
    #entries%changelogentry#
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