Emacs support: add hg-revert-buffer.

Fix callers to file.revision to use file.read This was causing unchanged files to show up as modified.

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