[PATCH 1 of 2 V2] extdiff: move external tool command line building into separate function

[Yuya Nishihara]

On Thu, 10 Jan 2019 21:37:20 -0800, Ludovic Chabant wrote:
> # HG changeset patch
> # User Ludovic Chabant <ludovic at chabant.com>
> # Date 1547180523 28800
> #      Thu Jan 10 20:22:03 2019 -0800
> # Node ID ef0e2f7224358c32b0f62b13e83e89ba2399c8cf
> # Parent  e546c124217485f54b897d050a9573fc4ab97ab7
> extdiff: move external tool command line building into separate function

Queued the first patch, thanks.