merge-recursive: rename merge_options argument from 'o' to 'opt'

The name 'o' was used for the merge_options struct pointer taken by many
functions, but in a few places it was named 'opt'.  Several functions
that didn't need merge_options instead used 'o' for a diff_filespec
argument or local.  Some functions needed both an inconsistently either
renamed the merge_options to 'opt' or the diff_filespec to 'one'.  I
want to remove the weird split in the codebase between using a
diff_filespec and a pair of (oid,mode) values in favor of using a
diff_filespec everywhere, but that dramatically increases the number of
cases where we want to use 'o' as a diff_filespec.  Rename the
merge_options argument to 'opt' to make room.

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

Author: newren