merge-recursive: rename diff_filespec 'one' to 'o'

merge-recursive: rename diff_filespec 'one' to 'o'

In the previous commit, we noted that several places throughout merge
recursive both had a reason to use 'o'; some for a merge_options struct,
and others for a diff_filespec struct.  Some places had both, forcing
one of the two to be renamed, though the choice was inconsistent.  Now
that the merge_options struct has been renamed to 'opt' everywhere, we
can replace the few places that used 'one' for the diff_filespec to 'o'.

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