[PATCH 1 of 2 V2] histedit: store backup file before histedit

[Matt Mackall]

On Mon, 2015-04-06 at 18:50 -0700, Durham Goode wrote:
> # HG changeset patch
> # User Durham Goode <durham at fb.com>
> # Date 1428140263 25200
> #      Sat Apr 04 02:37:43 2015 -0700
> # Node ID e511ce4fbc848c03986a2a8e740d153cf119b8d1
> # Parent  b2fb1403994e033584aed8a487ab162a9d75fa80
> histedit: store backup file before histedit
> 
> It's possible for the user to delete some of the commits they started with
> during a histedit, and aborting the histedit doesn't bring them back. Let's
> store a backup bundle so we can always recover the stack of commits from before
> they began.

I'm not thrilled with this because

- pickle needs to be burned with fire
- it's being extended to store a temporary filename
- that issue could be avoided by using a fixed bundle name

-- 
Mathematics is the supreme nostalgia of our time.