merge-recursive: comment and reorder the merge_options fields

merge-recursive: comment and reorder the merge_options fields

The merge_options struct had lots of fields, making it a little
imposing, but the options naturally fall into multiple different groups.
Grouping similar options and adding a comment or two makes it easier to
read, easier for new folks to figure out which options are related, and
thus easier for them to find the options they need.

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