Merge branch 'jc/maint-do-not-switch-to-non-commit' into maint

* jc/maint-do-not-switch-to-non-commit:
  git checkout: do not allow switching to a tree-ish that is not a commit

builtin-checkout.c

@@ -681,8 +681,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		argv++;
 		argc--;
 
+		new.name = arg;
 		if ((new.commit = lookup_commit_reference_gently(rev, 1))) {
-			new.name = arg;
 			setup_branch_path(&new);
 			if (resolve_ref(new.path, rev, 1, NULL))
 				new.commit = lookup_commit_reference(rev);

t/t2011-checkout-invalid-head.sh

@@ -10,6 +10,10 @@ test_expect_success 'setup' '
 	git commit -m initial
 '
 
+test_expect_success 'checkout should not start branch from a tree' '
+	test_must_fail git checkout -b newbranch master^{tree}
+'
+
 test_expect_success 'checkout master from invalid HEAD' '
 	echo 0000000000000000000000000000000000000000 >.git/HEAD &&
 	git checkout master --