Fix confusing message from hg revert (issue332)
% mkdir sub1
% touch sub1/file1
% hg add sub1/
adding sub1/file1
% hg rev sub1
sub1: No such file in rev e4c586763258
forgetting sub1/file1
% file replaced with directory
adding a
% should fail - would corrupt dirstate
abort: file named 'a' already in dirstate
% directory replaced with file
adding a/a
% should fail - would corrupt dirstate
abort: directory named 'a' already in dirstate
% directory replaced with file
adding b/c/d
% should fail - would corrupt dirstate
abort: directory named 'b' already in dirstate