bug#58516: 28.1; add option/key to rerun current diff without whitespace changes

[gert]

Sometimes, for example when you work together with others, there are meaningless whitespace changes in the diff.

In order to review these diff easier add a key to rerun the current diff without showing whitespace changes.

There is

   ("\C-c\C-w" . diff-ignore-whitespace-hunk)

which is useful, but incovenient to use when there are many hunks in the diff.

Add  a key which does the same, but for all hunks (e.g. diff-toggle-whitespace-changes), so you can easily switch a diff buffer to one which ignores all whitespace changes.