checkout: Add an option to require hardlinks

I've seen a few people hit this and wonder why checkouts are slow/take
space.  Really, ensuring this happens is the *point* of OSTree.
Physical copies should be a last resort fallback for very unusual
situations (one of those is rpm-ostree checking out the db since
librpm doesn't know how to read from libostree).

Even I hit the fact that `/var` is a mountpoint disallowing hardlinks
with `/ostree` once and was confused.  =)

Add this to the rofiles-fuse test case because it creates a mount
point.

Closes: #368
Approved by: jlebon

tests/test-rofiles-fuse.sh

@@ -26,7 +26,7 @@ skip_without_user_xattrs
 
 setup_test_repository "bare-user"
 
-echo "1..5"
+echo "1..6"
 
 mkdir mnt
 
@@ -71,3 +71,13 @@ echo "ok deletion"
 ${CMD_PREFIX} ostree --repo=repo commit -b test2 -s fromfuse --link-checkout-speedup --tree=dir=checkout-test2
 
 echo "ok commit"
+
+${CMD_PREFIX} ostree --repo=repo checkout -U test2 mnt/test2-checkout-copy-fallback
+assert_file_has_content mnt/test2-checkout-copy-fallback/anewfile-for-fuse anewfile-for-fuse
+
+if ${CMD_PREFIX} ostree --repo=repo checkout -UH test2 mnt/test2-checkout-copy-hardlinked 2>err.txt; then
+    assert_not_reached "Checking out via hardlinks across mountpoint unexpectedly succeeded!"
+fi
+assert_file_has_content err.txt "Invalid cross-device link"
+
+echo "ok checkout copy fallback"