Phillip Wood [off-list ref] writes:

quoted

@@ -800,6 +813,15 @@ static int rebase_config(const char *var, const char *value, void *data)
  		return 0;
  	}
  
+	if (!strcmp(var, "rebase.rebasemerges")) {
+		opts->config_rebase_merges = git_parse_maybe_bool(value);
+		if (opts->config_rebase_merges < 0) {
+			opts->config_rebase_merges = 1;
+			parse_rebase_merges_value(opts, value);
+		}

I think we need

	} else {
		opts->rebase_cousins = 0;
	}

here. Otherwise if rebase.rebaseMerges is set twice we wont follow the 
usual "last config wins" convention. For example

	[rebase]
		rebaseMerges=rebase-cousins
		rebaseMerges=true

will result in us unexpectedly rebasing cousins

Ah, good catch.

An alternative might be to use the "lookup" functions (e.g.
repo_config_get_maybe_bool()), which already implement "last one wins"
semantics. It's a bigger change, but it may be easier for future readers
to understand. I don't have a strong opinion on which is the 'better'
approach.