merge-ort: add some more explanations in collect_merge_info_callback()

merge-ort: add some more explanations in collect_merge_info_callback()

The previous patch possibly raises some questions about whether
additional cases in collect_merge_info_callback() can be handled early.
Add some explanations in the form of comments to help explain these
better.  While we're at it, add a few comments to denote what a few
boolean '0' or '1' values stand for.

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