Fixes <https://bugs.gnu.org/32161>.
Reported by Ricardo Wurmus <rekado@elephly.net>.
This mostly reverts 83099892e0.
* guix/store/deduplication.scm (counting-wrapper-port): New procedure.
(nar-sha256): Use it.
Previously they'd always be placed next to TO-REPLACE, which would lead
to EPERM in some cases.
* guix/store/deduplication.scm (replace-with-link): Add #:swap-directory
parameter and honor it. Add call to 'make-file-writable'. Catch
'system-error' around 'rename-file'.
(deduplicate): Pass #:swap-directory and remove uses of
'false-if-system-error'.
* tests/store-deduplication.scm ("deduplicate"): Add 'chmod' call.
* 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.
Ludovic Courtès