-			; /* default mode; do nothing */
+			warning(_("--rebase-merges with an empty string "
+				  "argument is deprecated and will stop "
+				  "working in a future version of Git. Use "
+				  "--rebase-merges=no-rebase-cousins "
+				  "instead."));

Just a small nit on the warning message. It should describe to the user
what behavior is happening here and presumably the user wanted
`--rebase-merges` with no argument. For example, "Defaulting to
--rebase-merges instead."