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

removing b
this update spans a branch affecting the following files:
 b
aborting update spanning branches!
(use update -m to merge across branches or -C to lose changes)