merge-ort: implement a very basic collect_merge_info()

merge-ort: implement a very basic collect_merge_info()

This does not actually collect any necessary info other than the
pathnames involved, since it just allocates an all-zero conflict_info
and stuffs that into paths.  However, it invokes the traverse_trees()
machinery to walk over all the paths and sets up the basic
infrastructure we need.

I have left out a few obvious optimizations to try to make this patch as
short and obvious as possible.  A subsequent patch will add some of
those back in with some more useful data fields before we introduce a
patch that actually sets up the conflict_info fields.

Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>