guix gc: Add '--optimize'.

* guix/scripts/gc.scm (show-help, %options): Add --optimize.
  (guix-gc): Handle it.

doc/guix.texi

@@ -1472,6 +1472,17 @@ Attempt to delete all the store files and directories specified as
 arguments.  This fails if some of the files are not in the store, or if
 they are still live.
 
+@item --optimize
+@cindex deduplication
+Optimize the store by hard-linking identical files---this is
+@dfn{deduplication}.
+
+The daemon performs deduplication after each successful build or archive
+import, unless it was started with @code{--disable-deduplication}
+(@pxref{Invoking guix-daemon, @code{--disable-deduplication}}).  Thus,
+this option is primarily useful when the daemon was running with
+@code{--disable-deduplication}.
+
 @item --list-dead
 Show the list of dead files and directories still present in the
 store---i.e., files and directories no longer reachable from any root.

guix/scripts/gc.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -43,6 +43,8 @@ Invoke the garbage collector.\n"))
                          collect at least MIN bytes of garbage"))
   (display (_ "
   -d, --delete           attempt to delete PATHS"))
+  (display (_ "
+      --optimize         optimize the store by deduplicating identical files"))
   (display (_ "
       --list-dead        list dead paths"))
   (display (_ "
@@ -88,6 +90,10 @@ Invoke the garbage collector.\n"))
                 (lambda (opt name arg result)
                   (alist-cons 'action 'delete
                               (alist-delete 'action result))))
+        (option '("optimize") #f #f
+                (lambda (opt name arg result)
+                  (alist-cons 'action 'optimize
+                              (alist-delete 'action result))))
         (option '("list-dead") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'action 'list-dead
@@ -169,6 +175,8 @@ Invoke the garbage collector.\n"))
          (list-relatives requisites))
         ((list-referrers)
          (list-relatives referrers))
+        ((optimize)
+         (optimize-store store))
         ((list-dead)
          (for-each (cut simple-format #t "~a~%" <>)
                    (dead-paths store)))