It's also okay if an expected tree object is actually a commit.

...that happens with submodules sometimes, so don't panic.

Reported by Sum-Wai Low.

git-subtree.sh

@@ -397,7 +397,7 @@ subtree_for_commit()
 	git ls-tree "$commit" -- "$dir" |
 	while read mode type tree name; do
 		assert [ "$name" = "$dir" ]
-		assert [ "$type" = "tree" ]
+		assert [ "$type" = "tree" -o "$type" = "commit" ]
 		echo $tree
 		break
 	done