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
% init
% commit
adding a
% annotate -c
8435f90966e4: a
% annotate -d
Thu Jan 01 00:00:01 1970 +0000: a
% annotate -n
0: a
% annotate -u
nobody: a
% annotate -cdnu
nobody 0 8435f90966e4 Thu Jan 01 00:00:01 1970 +0000: a