t1415: set REFFILES for test specific to storage format

Packing refs (and therefore checking that certain refs are not packed)
is a property of the packed/loose ref storage. Add a comment to explain
what the test checks.

Signed-off-by: Han-Wen Nienhuys <[email protected]>
Reviewed-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: hanwen

t/t1415-worktree-refs.sh

@@ -16,7 +16,10 @@ test_expect_success 'setup' '
 	git -C wt2 update-ref refs/worktree/foo HEAD
 '
 
-test_expect_success 'refs/worktree must not be packed' '
+# The 'packed-refs' file is stored directly in .git/. This means it is global
+# to the repository, and can only contain refs that are shared across all
+# worktrees.
+test_expect_success REFFILES 'refs/worktree must not be packed' '
 	git pack-refs --all &&
 	test_path_is_missing .git/refs/tags/wt1 &&
 	test_path_is_file .git/refs/worktree/foo &&