contrib: don't hardcode path to bash interpreter

Use the env binary to figure out the correct bash to use.
Certain systems ships with an ancient version of bash, but the
user might have installed a newer one that is earlier in $PATH.

For example the current version of Mac OS X ships version 3.2.51
of bash, which does not understand new fancy builtins such as
readarray. A user might install a newer version of bash, use that
as their shell and add that path before bin.

contrib/editmerge

@@ -1,4 +1,4 b''
-#!/bin/bash
+#!/usr/bin/env bash
 # A simple script for opening merge conflicts in the editor.
 # Use the following Mercurial settings to enable it.
 #

contrib/revsetbenchmarks.sh

@@ -1,4 +1,4 b''
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Measure the performance of a list of revsets against multiple revisions
 # defined by parameter. Checkout one by one and run perfrevset with every