> But could you explain the case when the changes to 'vc-diff-internal' are
> going to be used? If those are only for log-edit-show-diff, I think it'd be
> better if the new logic was implemented in the new value of
> log-edit-diff-function, rather than having it spliced into the common code
> path. Would that result in a lot of code duplication?

Not much code duplication, thanks for the suggestion, the patch below
sets log-edit-diff-function for log-edit-show-diff.

> It might also be worth it to thread the 'patch-buffer' value through the
> backend method arguments (the actual value will be the patch string), so
> that vc-git-checkin gets it in the 4th argument, rather than having it call
> (derived-mode-p 'diff-mode) (this feels a little brittle: I suppose which
> buffer is current during this call might change in the future). It would
> also automatically weed out backends which don't support this feature,
> rather than having an attempt to commit from a diff buffer using Hg fail
> silently.

I agree that in the first version ‘(derived-mode-p 'diff-mode)’ was brittle.
But changing the established API with a new argument doesn't look right.
So the next version below uses the buffer-local variable ‘vc-patch-string’.
In other backends such as Hg it shouldn't fail silently, but it will be
just less granular, and will commit whole files instead of edited diffs.

There is a new problem however.  After starting vc-log-edit from *vc-diff*,
and using log-edit-show-diff, it reuses the original buffer *vc-diff*.
This is not a problem, because the buffer-local variable ‘vc-patch-string’
is saved in the *vc-log* buffer.  But after deleting *vc-diff*, log-edit-done
fails on the deleted vc-parent-buffer with

  Debugger entered--Lisp error: (error "Selecting deleted buffer")
    vc-finish-logentry()
    funcall-interactively(vc-finish-logentry)
    log-edit-done()

But this is an old problem.  The same error is signaled
after typing ‘v’ in *vc-dir* buffer, then deleting the
original *vc-dir* buffer, and trying to type ‘C-c C-c’
(log-edit-done) in the *vc-log* buffer.