Alex Henrie [off-list ref] writes:

The unusual syntax --rebase-merges="" (that is, --rebase-merges with an
empty string argument) has been an undocumented synonym of
--rebase-merges=no-rebase-cousins.

[...]

 	if (rebase_merges) {
 		if (!*rebase_merges)
-			; /* 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."));
 		else if (!strcmp("rebase-cousins", rebase_merges))
 			options.rebase_cousins = 1;
 		else if (strcmp("no-rebase-cousins", rebase_merges))

As I mentioned in my review of patch 3/3, I think we might be better
served by saying that --rebase-merges="" is a synonym of --rebase-merges
(aka give me a sane default) instead of giving a specific value like
"no-rebase-cousins". This would be give us leeway to change the default
behavior in the future.