use bash

Signed-off-by: Etienne Millon <me@emillon.org>

test/blackbox-tests/test-cases/exec-prefix.t

@@ -24,7 +24,7 @@ The executable just displays "Hello" and its arguments.
 The wrapper parses its own arguments and executes the rest.
 
   $ cat > wrap.sh << 'EOF'
-  > #!/bin/sh
+  > #!/bin/bash
   > while getopts "xy" o; do
   >   echo "Got option: $o"
   >   shift $((OPTIND-1))