checkout: Don't hardlink zero sized files

Alternative to https://github.com/ostreedev/ostree/pull/2197

Python's (usually) zero-sized `__init__.py` files can provoke
us hitting the hardlink limits on some filesystems (`EMLINK`).
At least one Fedora rpm-ostree user hit this.

The benefits of hardlinking here are quite marginal; lots
of hardlinks can behave suboptimally in particular filesystems
like BTRFS too.

This builds on prior code which made this an option, introduced
in https://github.com/ostreedev/ostree/commit/673cacd633f9d6b653cdea530657d3e780a41bbd
Now we just do it uncondtionally.

Also this provoked a different bug in a very obscure user mode checkout
case; when the "real" permissions were different from the "physical"
permissions, we would still hardlink.  Fix the test case for this.

src/libostree/ostree-repo-checkout.c

@@ -637,8 +637,12 @@ checkout_one_file_at (OstreeRepo                        *repo,
 
       need_copy = FALSE;
     }
-  else if ((options->force_copy_zerosized && is_reg_zerosized) || override_user_unreadable)
+  else if (is_reg_zerosized || override_user_unreadable)
     {
+      /* In https://github.com/ostreedev/ostree/commit/673cacd633f9d6b653cdea530657d3e780a41bbd we
+       * made this an option, but in order to avoid hitting EMLINK, we now force copy zerosized
+       * files unconditionally.
+       */
       need_copy = TRUE;
     }
   else if (!options->force_copy)