bug#57400: 29.0.50; Support sending patches from VC directly

[Juri Linkov]

>>> I have done all of the above and am prepared to push the changes, but
>>> before I do I was wondering if anyone would have any objections to the
>>> following changes to vc-git.el:
>>
>> Indeed, abbreviated commits are more usable.  But please wait a little
>> for more comments.
>
> Also note that this change would also only affect working-revision and
> previous-revision.  If necessary or preferred, I could also modify the
> changes to only affect working-revision.

Nicer names would be better everywhere, including previous-revision.
But: In my recent mail to RMS I raised questions about 'C-u C-x v ='.
And when I tried your patch then the default value for "HEAD"
often is very strange.  For example, try to type 'C-u C-x v ='
on the file lisp/paren.el.  Here is what it shows:

  Older revision [remotes/origin/scratch/eglot2emacs~488]: 

I wonder where comes "eglot2emacs" from when the current branch is master?