Bug 1534500 - Run get_and_diffoscope with bash. r=dustin

This will allow to use bash constructs in pre-diff-commands, like
`{a,b}`.

Depends on D23075

Differential Revision: https://phabricator.services.mozilla.com/D23076

--HG--
extra : moz-landing-system : lando

taskcluster/docker/diffoscope/get_and_diffoscope

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 set -e
 set -x