Into a first half that determines what operation to do, and a second
half that does it.
Currently the only operation is diffing one or more commits, but a
later patch will add diffing of trees, at which point this refactoring
will pay off.
Signed-off-by: Karl Hasselström <kha@treskal.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>