selftests/signedoff-check.sh: run under BASH

The redirection on line 7 is not supported on minimalistic shells such
as DASH.  Let's force the use of BASH instead.

Signed-off-by: Cleber Rosa <[email protected]>

selftests/signedoff-check.sh

@@ -1,4 +1,4 @@
-#!/bin/sh -e
+#!/bin/bash -e
 
 echo "** Running signedoff-check..."
 if [ -z "$COMMIT_COUNT" ]; then