merge: shortcircuit filemerge for identical files
- use filectx.cmp to compare files
- move merge messages into filemerge
- kill the redundant resolving message
- update tests
% 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