Yes, I have tried the option you mention to edit the last commit with C-x
C-e and it is working fine.
But It seems that it only allows amending the last commit if I have edited
a file.

As I understand it I need to edit a file and then do vc-next-action to get
to the log-edit buffer
where you can do C-x C-e.

I could not find a way to do this if just the commit message is supposed to
be updated.

Den mån 25 nov. 2019 kl 12:48 skrev Dmitry Gutov <dgutov@yandex.ru>:

> On 23.11.2019 12:01, Fredrik Nyqvist wrote:
>
> > I would like to use vc mode to edit an outgoing commit message.
> >
> > Steps to reproduce:
> > 1. emacs -Q /path/to/git/repo
> > 2. Edit a file and then
> > 3. Run vc-next-action to create a commit: C-x v v
> > 4. Write a commit message and end with C-c C-c
> > 5. Now Open the outgoing log (vc-log-outgoing) : C-x v O
> > 6. Try to edit the commit message (log-view-modify-change-comment): e
> >
> > This message is printed:
> >
> > log-view-current-file: Unable to determine the current file
>
> Yes. This feature seems unfinished, and it was never updated for the
> newer VC systems.
>
> However, we have an option for amending the last commit (press C-x C-e
> in an already open log-edit buffer). It only works with Git and Hg now.
>

It would be good to unify these two features somehow.
>