> I don't know if users will find this too annoying, maybe not,
> but probably this is what should be done to solve such dilemma that
> the same diff can be used in two ways: by applying the diff to the
> buffers with 'C-c C-a', or by applying to the files with 'git apply'.
> Without saving the buffers before running the command, either
> will fail: 'C-c C-a' will fail on changes produced from files,
> 'git apply' will fail on changes produced from unsaved buffers.

Here is a new customizable option 'multi-file-diff-unsaved'
that defines what to do with unsaved changes.  When it is
'use-buffer' then it handles the case Eshel demonstrated
where changes are applied over unsaved buffers.  But when
the value is 'save-buffers', then 'save-some-buffers' is
called before producing the diff.