Clean up some merge logic
- rename mode to branch_merge
- use explicit update mode
- use negative mtime for updates that set mtime
- expand some cryptic variable names
- elaborate merge dirstate comments
- remove redundant manifest lookup for non-merge case
- remove impossible merge case
- fix up test cases
merging for b
merging b
This is file b1
merging for b
merging b
merging for b
merging b