completion: --set-upstream option for git-branch

Signed-off-by: NMichael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -797,6 +797,7 @@ _git_branch ()
 		__gitcomp "
 			--color --no-color --verbose --abbrev= --no-abbrev
 			--track --no-track --contains --merged --no-merged
+			--set-upstream
 			"
 		;;
 	*)