merge is always the current branch head. It is meant to
be used to supersede old development history of side
branches.
+
+subtree::
+ This is a modified recursive strategy. When merging trees A and
+ B, if B corresponds to a subtree of A, B is first adjusted to
+ match the tree structure of A, instead of reading the trees at
+ the same level. This adjustment is also done to the common
+ ancestor tree.