merge-ort: record stage and auxiliary info for every path
Create a helper function, setup_path_info(), which can be used to record
all the information we want in a merged_info or conflict_info. While
there is currently only one caller of this new function, and some of its
particular parameters are fixed, future callers of this function will be
added later.
Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>