A diff may contain just headers and no hunk.  When washing a diff
directly, we already detected that by checking whether we have reached
the beginning of the next diff or eob.

When the diff is part of a log, we additionally have to also check
whether we have reached the next log heading.

Alternatively we could pass along the limit from `magit-log-wash-rev'
(as a marker, because we modify the buffer text), but that would
complicate matters.

Closes #5454.

As for `completing-read-multiple', if REQUIRE-MATCH is t, then all
inputs must appear in the collection, but it is again possible to
select nothing.

Forge's uses of this function need this behavior.  In [1: 4e2dd7ba6f]
I should only have added the new `any' value, instead of also changing
the meaning of t.

Closes magit/forge#814.

1: 2025-08-16 4e2dd7ba6f54b2251a702b4c0416467d7315407a
   magit-completing-read-multiple: Support requiring non-empty input

Generally, when we bind SPC to some "scroll down" command, we also
bind DEL *and* S-SPC to the respective "scroll up" command.  Do the
same here.

Before the menu `magit-revision-jump' was added, when point is on a
file in a diff "j" (`magit-jump-to-diffstat-or-diff') jumped to the
respective file within the diffstat.  For `magit-diff-mode' it still
does that.

In `magit-revision-mode' "j" is now bound to `magit-revision-jump',
with partially shadows the above behavior.  To continue to allow
quickly jumping between a file in a revision diff and the same file
listed in the diffstat, `magit-revision-jump' pivots to calling
`magit-jump-to-diffstat-or-diff' when invoked in those locations.

The hope was that would preserve enough of the old behavior to make
everyone happy.  I.e., on a file jump to the other place that file
is listed (as before), everywhere else (or with a prefix argument)
show the menu, letting the user explicitly choose where to jump to
this time around (previously this would have jumped to the diffstat).

Users who want the old behavior back can do:

  (keymap-unset magit-revision-mode-map "j" t)

Instead of adding a more explicit knob to restore the old behavior,
this commit improves a variation:

If `magit-jump-revision-diffstat' is invoked on a hunk (via "j s"),
that now goes to the respective file in the diffstat, instead of to
the heading of the diffstat itself.

The hope is that going to a more precise location outweighs the added
cost of having to press "j s" instead of just "j" to go to the diffstat.

Do the same for `magit-jump-to-revision-diffstat', so that those users,
who remove the new binding in `magit-revision-mode-map', can benefit
too.

Now let's just hope nobody dislikes going to the file heading instead of
to the diffstat heading.

Closes #5442.

The next commit will make changes that `magit-define-section-jumper'
does not support.