grep: extend functionality, add man page entry, add unit test. walkchangerevs now returns a two-tuple. Its behaviour is also extensively commented. The annotate command's getname function has been factored out to a new function, trimname, so it can be shared between annotate and grep. The behaviour of grep has been beefed up, so that it now performs a number of useful functions.

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    <title>#repo|escape# Changelog</title>
    <description>#repo|escape# Changelog</description>
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