submodule: trace adding submodule ODB as alternate

Submodule ODBs are never added as alternates during the execution of the
test suite, but there may be a rare interaction that the test suite does
not have coverage of. Add a trace message when this happens, so that
users who trace their commands can notice such occurrences.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -201,6 +201,8 @@ int register_all_submodule_odb_as_alternates(void)
 		add_to_alternates_memory(added_submodule_odb_paths.items[i].string);
 	if (ret) {
 		string_list_clear(&added_submodule_odb_paths, 0);
+		trace2_data_intmax("submodule", the_repository,
+				   "register_all_submodule_odb_as_alternates/registered", ret);
 		if (git_env_bool("GIT_TEST_FATAL_REGISTER_SUBMODULE_ODB", 0))
 			BUG("register_all_submodule_odb_as_alternates() called");
 	}

t/README

@@ -456,11 +456,8 @@ GIT_TEST_FATAL_REGISTER_SUBMODULE_ODB=<boolean>, when true, makes
 registering submodule ODBs as alternates a fatal action. Support for
 this environment variable can be removed once the migration to
 explicitly providing repositories when accessing submodule objects is
-complete (in which case we might want to replace this with a trace2
-call so that users can make it visible if accessing submodule objects
-without an explicit repository still happens) or needs to be abandoned
-for whatever reason (in which case the migrated codepaths still retain
-their performance benefits).
+complete or needs to be abandoned for whatever reason (in which case the
+migrated codepaths still retain their performance benefits).
 
 Naming Tests
 ------------