checkout: Fix bare-user symlink checkouts

Logic error introduced after refactoring; we hoisted the
`is_bare_user_symlink` variable to the top, but its computation
below.  But the `is_bare` symlink depended on it.

Closes: https://github.com/ostreedev/ostree/issues/798

Closes: #799
Approved by: jlebon

tests/basic-test.sh

@@ -44,6 +44,7 @@ validate_checkout_basic() {
      assert_has_file baz/cow
      assert_file_has_content baz/cow moo
      assert_has_file baz/deeper/ohyeah
+     assert_symlink_has_content somelink nosuchfile
      )
 }