builtin-apply.c: clean-up apply_one_fragment()

builtin-apply.c: clean-up apply_one_fragment()

We had two pointer variables pointing to the same buffer and an
integer variable used to index into its tail part that was
active (old, oldlines and oldsize for the preimage, and their
'new' counterparts for the postimage).

To help readability, use 'oldlines' as the allocated pointer,
and use 'old' as the pointer to the tail that advances while the
code builds up the contents in the buffer.  The size 'oldsize'
can be computed as (old-oldines).

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