bdiff: normalize the diff (issue1295) When the common part of a diff can be moved forward, move it forward. Otherwise we don't get deterministic results (it would depends on the way we split for the recursion). That way we get identical hunks when doing the same change, it helps to solve issue1295 (inconsistent diffs on different side during a merge).

foo renamed from bar:26d3ca0dfd18e44d796b564e38dd173c9668d3a9
   rev    offset  length   base linkrev nodeid       p1           p2
     0         0       6      0       0 26d3ca0dfd18 000000000000 000000000000
     1         6       7      1       1 d267bddd54f7 26d3ca0dfd18 000000000000