Adding `vc-figcmd` to mirror `vc-hgcmd`, like `vc-fig`.

[Daniel Pittman]

G'day.  I'm planning to produce a version of `vc-fig` that uses the `vc-hgcmd` backend rather than `vc-hg`, as an experiment.

Assuming it works correctly do y'all have a strong opinion if I should:

1. send it as a replacement for `vc-fig`, or
2. send it as `vc-figcmd` to live next to it, or
3. see about a configuration option to make `vc-fig` support both backends, or
4. just keep it to myself?

My initial testing says that it will work, but I need to use it further to figure out if it does the right stuff.  

As with the upstream of `vc-hgcmd` the main advantages of the new backend would be speed, since a persistent Mercurial process will be used to run commands.  

An additional bonus is support for a handful more VC features introduced very recentel in Emacs, but see upstream for other caveats about the backend: https://github.com/muffinmad/emacs-vc-hgcmd