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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# should print nothing

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# should print ? b/x

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? b/x

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# should print A b/x

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A b/x

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# should forget b/x

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forgetting b/x

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# should print nothing