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

changeset:   0:9426b370c206
tag:         tip
user:        My Name <myname@example.com>
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     commit-1