On main, if I run
$ make regress GOT_TEST_PACK=1
I get:

===> regress
===> regress/cmdline
./checkout.sh -q -r "/tmp"
./update.sh -q -r "/tmp"
   .
   .
   .
./fetch.sh -q -r "/tmp"
./tree.sh -q -r "/tmp"
--- /tmp/got-test-tree_submodule_of_same_repo-eGrVPmMM/stderr.expected
   Tue Dec  8 09:59:48 2020
+++ /tmp/got-test-tree_submodule_of_same_repo-eGrVPmMM/stderr    Tue
Dec  8 09:59:48 2020
@@ -1,2 +1,2 @@
-got-read-tree: bad object data
+got-read-pack: bad object data
 got: bad object data
FAILED
   .
   .
   .