deduplicate: Fix a couple of thinkos.

deduplicate: Fix a couple of thinkos.

* guix/store/deduplication.scm (get-temp-link): Turn 'args' in the 'catch'
handler into a rest argument.
(deduplicate): Use 'lstat' instead of 'file-is-directory?' to properly
handle symlinks.  When iterating over the result of 'scandir', exclude
the ".links" sub-directory.
* tests/store-deduplication.scm ("deduplicate"): Create sub-directories
and call 'deduplicate' directly on STORE.