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

[Eli Zaretskii]

> From: Philip Kaludercic <philipk@posteo.net>
> Cc: Antoine Kalmbach <ane@iki.fi>,  57400@debbugs.gnu.org
> Date: Fri, 26 Aug 2022 11:26:29 +0000
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >>   5. Loop each file in `message-mode`. `C-c C-c` sends and goes to the
> >>      next patch, `C-c C-k` cancels the whole thing.
> >
> > Please don't hard-code message-mode.  Please honor the user setting of
> > mail-user-agent instead.
> 
> Is there a generic way to handle any mail-user-agent?

For some value of "generic way to handle", yes.  For example,
compose-mail does that.

> E.g. if you want to add attachments, is there any better way that
> just doing a case distinction on known user agent implementations?

Not sure about attachments, but we could either add another property
to mail-user-agent, like we do with composefunc property, or dispatch
on the agent itself.

> > Also, I'm not sure why we'd need to send each patch file separately.
> > Why not add them one by one as attachments to the same email message?
> 
> This wouldn't work if we are sending patches to a mailing list that
> assumes patches are sent out by git send-email, and that the messages
> can be filtered through git am.

"git am" handles attachments without any problems, I do it all the
time.

But I don't object to having optional behaviors here.  My point is
that we should allow sending all the patches together, as that is the
preferred/usual practice in Emacs development.