git-prompt: fix uninitialized variable

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>