git-hooks: post-merge script does not need /bin/bash

Just use /bin/sh, which exists on more systems.

Change-Id: I5e5453b89841510473414a4c339b647b4f95ae7b

.git-hooks/post-merge

-#!/bin/bash
+#!/bin/sh
 
 # Do not warn if there were no real merge
 git rev-parse -q --verify HEAD^2 >/dev/null || exit