Short story of how I got to this bug report:

For a while now, I've been wanting to contribute to Emacs by doing
something more than reporting bugs and try to provide trivial fixes for
the bugs I found.  So I looked into emacs-devel, to read about
recomendations for beginning to contribute.  Based on some of the mails
I found [1], it looks like working on some wishlist items or minor bugs
would be good.  So I said to myself: OK, let's see what is the oldest
one still open.  And here I am.


Now I'll make a summary of what I've understood from the bug report:

The wish is to have a command that would act like 'revert-buffer'
(e.g., modifying the buffer rightaway), but that tries to do a better job
preserving markers.  Also, it is desirable that undo info is preserved.
It is known that 'revert-buffer' preserves undo info nowadays, but what I
understand is that it would be good to keep undo info of parts of the
total reverted change, so the undo of the reverted action can be made by
steps, and not as a single undo operation.

I think that the alternatives proposed after the report [2] do not
fulfill the wish because they do not modify the buffer immediately.  I'm
not sure how they act in regards to preserving markers and the undo
info, though.

AFAIK, the functionality wanted is still not present, so I'd guess it's
still relevant to work in this matter.

So for a week or so now, I've been working in a command that does what I
think it is wanted.  The command is called 'revert-buffer-by-hunks'
(I've added 'rbbh' as a prefix for sending the file for you to
see/test), because it calls 'diff' and then with the output patches the
buffer.

At first, I wrote a command that used diff-mode under the hood, but I've
been having troubles with preserving markers.  So I took a step back,
and wrote a new function that patches the buffer with the contents of
the file visited on disk.  With that done, I think it is time for me to
show what I've written so far, in order to:

1) Know if the functionality is present (I don't think so, but I believe
this is the first thing to know in order to advance).
2) There's still interest in having this command.
3) Know if working on this subject would be appreciated, or should I
move on to other things.
4) Receieve feedback, suggestions, fixes on things I'm sure I'm missing.
5) Discuss some of the questions that have arisen while I've been
working on this.

I'll wait for answers regarding to 1-4.  With regards to 5, I would like
to read opinions about:
a) What variables would you think should be customizable?
b) After reverting by hunks, I think it would be desirable to navigate
through the hunks reverted and toggle their state (i.e., go back and
forth to the contents before the revert operation and the contens on
disk).  That is because I think it would be kinda annoying to want to
undo some of the reverted hunks, and to do that having to undo
sequentially from the Nth hunk reverted to the desired one.

I attach a first draft of my work.  I'm hoping to hear suggestions and
corrections to improve it.

Best regards,
Mauro.

[1]
http://lists.gnu.org/archive/html/emacs-devel/2018-07/msg00451.html
http://lists.gnu.org/archive/html/emacs-devel/2017-08/msg00675.html
http://lists.gnu.org/archive/html/emacs-devel/2013-10/msg00805.html

[2]
Command diff-hl-revert-hunk, from diff-hl package
Command diff-buffer-with-file
Command ediff-current-file