* New command `nix-store --optimise' to reduce Nix store disk space

usage by finding identical files in the store and hard-linking them
  to each other.  It typically reduces the size of the store by
  something like 25-35%.  This is what the optimise-store.pl script
  did, but the new command is faster and more correct (it's safe wrt
  garbage collection and concurrent builds).

src/nix-store/help.txt

@@ -21,6 +21,7 @@ Operations:
 
   --init: initialise the Nix database
   --verify: verify Nix structures
+  --optimise: optimise the Nix store by hard-linking identical files
 
   --version: output version information
   --help: display help