merge-ort: add function grouping comments

merge-ort: add function grouping comments

Commit b658536f59 ("merge-ort: add some high-level algorithm structure",
2020-10-27) added high-level structure of the ort merge algorithm.  As
we have added more and more functions, that high-level structure has
been slightly obscured.  Since functions are still grouped according to
this high-level structure, add comments denoting sections where all the
functions are specifically tied to a piece of the high-level structure.

This function groupings include a few sub-divisions of the original
high-level structure, including some sub-divisions that are yet to be
submitted.  Each has (or will have) several functions all serving as
helpers to one or two main functions for each section.

As an added bonus, the comments will serve to provide a small textual
separation between nearby sections and allow the next three patch series
to be submitted independently and merge cleanly.

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