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

[Richard Stallman]

[[[ To any NSA and FBI agents reading my email: please consider    ]]]
[[[ whether defending the US Constitution against all enemies,     ]]]
[[[ foreign or domestic, requires you to follow Snowden's example. ]]]

    > 1. `M-x vc-prepare-patch`
    > 2. Dispatch to `vc-git-prepare-patch`

That would happen if the file uses git,
Please implement a default alternative that works generically
for other VC systems.

    > 3. Git wants a revision range, so interactively prompt for that
    >    (e.g. `HEAD^`, `abcd1234..ghjk5678`, or `-1`)
    > 4. `call-process` to `git format-patch $REV`, and so forth, get the
    >    list of files.
    > 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.

`message-mode' is one of Emacs's packages for composing mail.  Please
use `compose-mail' so that you use whichever one the user has selected.

I think you can insert the patch text into the composition buffer
after `compose-mail' returns, and it will work with either of the two
usual mail composition packages.

-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)