Add new file status: CONFLICT
Allow working copy write to support conflict and modify

* Before would just write all modified files as conflicts.
* readMergeDiff now rewrites MODIFY statuses to CONFLICT
Test merge when file added by branch being merged in
Handle conflict status in main file status diff fn
Make merge index writer write modified (no conflict) files correctly
Test merge merges modified non conflicting files
Use readCommitToc to read head tree in merge tests