test: Fix shellcheck by giving git-hashing scripts shebangs

This seems to be the way to do it now, even though I can't run them
without setting at least one env var.
I'll only fix shellcheck for now. Don't shoot the messenger.

It isn't quite clear to me why the previous commit masked this problem,
but I'm glad shellcheck has an effect or more effect now.

Author: roberth

tests/functional/git-hashing/fixed.sh

@@ -1,3 +1,5 @@
+#!/usr/bin/env bash
+
 source common.sh
 
 repo="$TEST_ROOT/scratch"