merge-recursive: remove final remaining caller of merge_file_one()

merge-recursive: remove final remaining caller of merge_file_one()

The function names merge_file_one() and merge_file_1() aren't
particularly intuitive function names, especially since there is no
associated merge_file() function that these are related to.  The
previous commit showed that merge_file_one() was prone to be called when
merge_file_1() should be, and since it is just a thin wrapper around
merge_file_1() anyway and only has one caller left, let's just remove
merge_file_one() entirely.

(It also turns out that the one remaining caller of merge_file_one()
has very broken code that needs to be completely rewritten, but that's
the subject of a future patch series; for now, we're just translating
it into a merge_file_1() call.)

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