Be consistent in choice of shell across all shell scripts in repo

tools/mockup.sh

@@ -1,4 +1,4 @@
-#! /bin/sh
+#!/bin/bash
 
 if [ $# -eq 0 ]; then
     # With no args, read stdin to scrape compiler output.

tools/rel.sh

@@ -1,4 +1,4 @@
-#! /bin/sh
+#!/bin/bash
 
 from=${1}
 to=${2}

tools/update-mocks.sh

@@ -1,4 +1,4 @@
-#! /bin/sh
+#!/bin/bash
 # Script to rewrite the autogenerated mocks in a unit test between
 # /* AUTOGENERATED MOCKS START */ and /* AUTOGENERATED MOCKS END */
 # based on link failures.