merge-recursive: switch from (oid,mode) pairs to a diff_filespec

There was a significant inconsistency in the various parts of the API
used in merge-recursive; many places used a pair of (oid, mode) to track
file version/contents, while other parts used a diff_filespec (which
have an oid and mode embedded in it).  This inconsistency caused lots of
places to need to pack and unpack data to call into other functions.
This has been the subject of some past cleanups (see e.g. commit
0270a07 ("merge-recursive: remove final remaining caller of
merge_file_one()", 2018-09-19)), but let's just remove the underlying
mess altogether by switching to use diff_filespec.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>