Make it return copied source and insertion separately, so that
later implementation of heuristics can use them more flexibly.

This does not change the heuristics implemented in
diffcore-rename nor diffcore-break in any way.
Signed-off-by: NJunio C Hamano <junkio@cox.net>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>