fix incorrect warning when walking on a particular rev
when a directory was given instead of a file it reported
incorrectly 'No such file or directory in rev <rev>'
we test if the file is a prefix directory
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<title>Mercurial repositories index</title>
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<h2>Mercurial Repositories</h2>
The specified repository "#repo|escape#" is unknown, sorry.
Please go back to the main repository list page.
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