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

[Lars Ingebrigtsen <larsi@gnus.org>]

Juri Linkov <juri@linkov.net> writes:

> Currently 'C-x v =' (vc-diff) can be easily mistyped as 'C-x v +' (vc-pull)
> often with an adverse effect because '=' and '+' are on the same key.
> It would be nice to bring both 'vc-pull' and 'vc-push' under the same
> prefix key 'C-x v p'.  And to add vc-prepare-patch under the same prefix.

Personally, I've never hit `C-x v +' when I meant to hit `C-x v =' --
because the latter is much easier to type.  (On a US keyboard, I'm sure
it varies.)

But grouping these commands more logically under `C-x v p' does sound
attractive.  That would also give us a place to put pull-and-then-push.
But:

> 'p' and 'S' in vc-dir are specific to the vc-dir buffer.
> So 'C-x v S' could be bound to the command vc-log-search
> that has no key binding.

If we want `C-x v p' as a prefix, then I think `p' should also be a
prefix in vc-dir.  But it's taken for a command that I think people
probably use all the time, so I don't think it's feasible to take it.