completion: add missing branch.*.rebase values

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1808,7 +1808,7 @@ _git_config ()
 		return
 		;;
 	branch.*.rebase)
-		__gitcomp "false true"
+		__gitcomp "false true preserve interactive"
 		return
 		;;
 	remote.pushdefault)