bug#43299: 28.0.50; message-newline-and-reformat does not insert space after citation prefix

[Amin Bandali]

This has been a pet peeve of mine for a while now, so I thought I'd send
in a report.

Currently, the `message-newline-and-reformat' function (bound to M-RET
in `message-mode') does not insert an empty space after the citation
prefix (e.g. '>') when reformatting the lines following the point in a
common use scenario.  I would like the behaviour to change, or at least
an option be added to have `message-newline-and-reformat' insert a space
after each '>'.

Example:

--8<---------------cut here---------------start------------->8---
> test0
>
> test1 test2 test3 test4 test5 test6 test7 test8 test9 test10 test11 test12
--8<---------------cut here---------------end--------------->8---

If you put the point on the line between test0 and test1, after the '>'
character, and press M-RET, it will result in:

--8<---------------cut here---------------start------------->8---
> test0
>



>
> test1 test2 test3 test4 test5 test6 test7 test8 test9 test10 test11
>test12
--8<---------------cut here---------------end--------------->8---

Notice the absence of space between ">" and "test12".  If the original
line is long enough for the filled version to span several lines, all of
them will not have a space after '>', similar to the above example.

Instead, I would like pressing M-RET in the above example to yield:

--8<---------------cut here---------------start------------->8---
> test0
>



>
> test1 test2 test3 test4 test5 test6 test7 test8 test9 test10 test11
> test12
--8<---------------cut here---------------end--------------->8---

As somewhat of a workaround, one could manually insert a space on the
line between "test0" and "test1" before calling the function, but that
results in the two middle lines (which only consist of '>') to have an
extraneous trailing space, i.e. "> ".