[PATCH 5 of 5 evolve-ext] prune: add --keep to not touch the working copy

[Jordi Gutiérrez Hermoso]

On Thu, 2015-03-19 at 18:14 -0700, Durham Goode wrote:
> # HG changeset patch
> # User Durham Goode <durham at fb.com>
> # Date 1426787762 25200
> #      Thu Mar 19 10:56:02 2015 -0700
> # Node ID 811a7345955e540045d57ed5698db0f12c27f36e
> # Parent  e93100523ef10fae3fcee2bdeac0f676ed842ecd
> prune: add --keep to not touch the working copy
> 
> In order to more closely emulate strip, lets add --keep to leave the working
> copy untouched when we do our prune.

We already have uncommit. I think it makes more sense to keep this as
two separate commands than trying to emulate the strip UI. Perhaps
give hints on how to alias uncommit + prune into a single verb.

Smaller, modular verbs instead of stuffing all the funcionality into a
single verb.