bug#62940: 29.0.60; vc: no easy way to get diff of all outgoing changes

[Spencer Baugh <sbaugh@janestreet.com>]

Reproduction:
1. Make some local changes and commits
2. C-x v O to see the list of local commits that will be pushed
3. But there's no quick way to get a diff combining all those changes,
which is useful if you've made multiple changes to the same function
over the course of multiple commits, or if you want to review your
changes overall before sending them out for review, or any other things
like that.

C-u C-x v = lets you specify two commits to diff, and the second commit
even defaults to the working revision, but there's no easy way to get
the "base" commit that is before all your local changes.  If you're
using git, you can try to remember your upstream branch and type that
in, but that's just a matter of memory.  For other VCSs (e.g. hg)
there's no easy way.

I'm working on adding an easy way to do this in vc, but there are
several ways to approach it and I wanted to check first.

A. (my preferred way) Support a M-1 prefix argument for C-x v =, which
will do a diff against the "base" revision, whatever that is.

B. Add a new keybinding in vc-read-revision which will insert the "base"
revision, so you can just use C-u C-x v = and then insert the base
revision at the first prompt.  Seems hard to make compatible with
non-default completion frameworks.

C. Add new equivalents to C-x v = and C-x v D which diff against the
base revision.  Seems like ugly duplication of keybindings.

D. Make some way to go from a log-view like C-x v O to a diff which
combines all those commits.  Could be nice, but seems a little
unintuitive, and also doesn't support including uncommitted changes in
the diff.

Thoughts, suggestions?