git-prompt: rename `upstream` to `upstream_type`

git-prompt: rename `upstream` to `upstream_type`

In `__git_ps1_show_upstream` rename the variable `upstream` to
`upstream_type`. This allows `__git_ps1_show_upstream` to reference a
variable named `upstream` that is declared `local` in `__git_ps1`, which
calls `__git_ps1_show_upstream`.

Signed-off-by: Justin Donnelly <justinrdonnelly@gmail.com>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>