[PATCH 1 of 2] revset: add minus revset class

[Durham Goode]

On 2/10/16 4:23 PM, Durham Goode wrote:
> # HG changeset patch
> # User Durham Goode <durham at fb.com>
> # Date 1455150122 28800
> #      Wed Feb 10 16:22:02 2016 -0800
> # Node ID edfc4fda0c1ca78186d9a779f0d4d0cfc6e1c03f
> # Parent  c791244507faabf86b1fd71746ec5b3d0364ed7d
> revset: add minus revset class
>
> Previously, revsets like 'X - Y' were translated to be 'X and not Y'. This can
> be expensive, since if Y is a single commit then 'not Y' becomes a huge set and
> sometimes the query optimizer doesn't account for it well.
>
> This patch adds a minusset smart revset class that knows how to handle the minus
> operator smartly. A future patch will change the optimizer to use this instead
> of changing '- Y' to be 'and not Y'.
>
> On a large repo this saves 2.2 seconds on rebase and histedit because "X:: - X"
> becomes almost instant. The future patch that actually uses the minus set will
> include more detailed performance numbers.
>
I forgot to mention it in the description, but this code is largely 
based on the existing addset class, with some optimizations removed that 
only apply to add cases.