More changes to support commits from non-single-revision trees:
More changes to support commits from non-single-revision trees:

(merge_args): Split `node' field into `ancestor_node' and
`source_node' fields.  Document all fields.
(txn_body_merge): Use new field.
(svn_fs_commit_txn): Initialize new field.
(svn_fs_merge): Start to implement.

Bug fix:

(merge): Use svn_fs__dag_delete_tree instead of svn_fs__dag_delete,
when deleting unmodified subtrees from the txn.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@838997 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion