git-prompt: fix uninitialized variable

First use is in the form: local var; ...; var=$var$whatever... If the variable was unset (as bash and others do after "local x"), then it would error if set -u is in effect. Also, many shells inherit the existing value after "local var" without init, but in this case it's unlikely to have a prior value. Now we initialize it. (local var= is enough, but local var="" is the custom in this file) Signed-off-by: Avi Halachmi (:avih) <avihpit@yahoo.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>