rerere: refactor rerere logic to make it independent from I/O

rerere: refactor rerere logic to make it independent from I/O

This splits the handle_file() function into in-core part and I/O
parts of the logic to create the preimage, so that we can compute
the conflict identifier without having to use temporary files.

Earlier, I thought the output from handle_file() should also be
refactored, but it is always about writing preimage (or thisimage)
that is used for later three-way merge, so it is saner to keep it
to always write to FILE *.

Signed-off-by: Junio C Hamano <gitster@pobox.com>