shell for gitprompt.sh needs to be bash

changed #!/bin/sh to #!/bin/bash as eg Debian and Ubuntu default
shell is /bin/dash. This script will not work with it, and so the
default shell has to be /bin/bash.

gitprompt.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 function async_run() {
   {