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
pulling from ../a
searching for changes
abort: repository is unrelated
pulling from ../a
searching for changes
warning: repository is unrelated
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg heads' to see heads, 'hg merge' to merge)
changeset: 1:bdcee5d51fa6
tag: tip
user: a
date: Mon Jan 12 13:46:40 1970 +0000
summary: a
changeset: 0:f155ba1aa5ba
user: b
date: Mon Jan 12 13:46:40 1970 +0000
summary: b