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

1 files updated, 0 files merged, 0 files removed, 0 files unresolved
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
abort: Permission denied - test-ro-message/b/vehicle