completion: complete branch name for "branch --set-upstream-to="

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

Author: Michael J Gruber

contrib/completion/git-completion.bash

@@ -875,6 +875,9 @@ _git_branch ()
 	done
 
 	case "$cur" in
+	--set-upstream-to=*)
+		__gitcomp "$(__git_refs)" "" "${cur##--set-upstream-to=}"
+		;;
 	--*)
 		__gitcomp "
 			--color --no-color --verbose --abbrev= --no-abbrev