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

#!/bin/sh

mkdir a
cd a
hg init
echo foo > b
hg add b
hg ci -m "b" -d "1000000 0"

chmod -w .hg

cd ..

hg clone a b

chmod +w a/.hg # let test clean up

cd b
hg verify