merge-recursive: avoid wrapper function when unnecessary and wasteful

merge-recursive: avoid wrapper function when unnecessary and wasteful

merge_file_one() is a convenience function taking a bunch of oids and
modes, combining each pair into a diff_filespec, and then calling
merge_file_1().  When we already start with diff_filespec's, we can
just call merge_file_1() directly instead of splitting out the oids
and modes for the wrapper to recombine into what we already had.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>