repo/checkout: Verify early if src/destination are on same device

At least in all Linux kernels up to today, one can never `link()` across
devices, so we might as well verify that up front. This will help for a future
patch to add a new type of union-add checkout, since Linux checks for `EEXIST`
before `EXDEV`.

Closes: #714
Approved by: jlebon

tests/test-rofiles-fuse.sh

@@ -78,6 +78,6 @@ assert_file_has_content mnt/test2-checkout-copy-fallback/anewfile-for-fuse anewf
 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"
+assert_file_has_content err.txt "Unable to do hardlink checkout across devices"
 
 echo "ok checkout copy fallback"