builtin/rebase.c: free() "options.strategy_opts"

When the "strategy_opts" member was added in ba1905a5fe (builtin
rebase: add support for custom merge strategies, 2018-09-04) the
corresponding free() for it at the end of cmd_rebase() wasn't added,
let's do so.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -1850,6 +1850,7 @@ cleanup:
 	free(options.gpg_sign_opt);
 	string_list_clear(&options.exec, 0);
 	free(options.strategy);
+	free(options.strategy_opts);
 	strbuf_release(&options.git_format_patch_opt);
 	free(squash_onto_name);
 	free(keep_base_onto_name);