completion: add --set-upstream-to and --unset-upstream

Remove --set-upstream as it's deprecated now.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -879,7 +879,8 @@ _git_branch ()
 		__gitcomp "
 			--color --no-color --verbose --abbrev= --no-abbrev
 			--track --no-track --contains --merged --no-merged
-			--set-upstream --edit-description --list
+			--set-upstream-to= --edit-description --list
+			--unset-upstream
 			"
 		;;
 	*)