branch: colour upstream branches

Otherwise when using 'git branch -vv' it's hard to see them among so
much output.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -807,7 +807,8 @@ color.branch::
 color.branch.<slot>::
 	Use customized color for branch coloration. `<slot>` is one of
 	`current` (the current branch), `local` (a local branch),
-	`remote` (a remote-tracking branch in refs/remotes/), `plain` (other
+	`remote` (a remote-tracking branch in refs/remotes/),
+	`upstream` (upstream tracking branch), `plain` (other
 	refs).
 +
 The value for these configuration variables is a list of colors (at most