nix store gc: account for auto-optimised store

Before the change on a system with `auto-optimise-store = true`:

    $ nix store gc --verbose --max 1

deleted all the paths instead of one path (we requested 1 byte limit).

It happens because every file in `auto-optimise-store = true` has at
least 2 links: file itself and a link in /nix/store/.links/ directory.

The change conservatively assumes that any file that has one (as before)
or two links (assume auto-potimise mode) will free space.

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

src/libstore/gc.cc

@@ -841,7 +841,8 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
             if (unlink(path.c_str()) == -1)
                 throw SysError("deleting '%1%'", path);
 
-            results.bytesFreed += st.st_size;
+            /* Do not accound for deleted file here. Rely on deletePath()
+               accounting.  */
         }
 
         struct stat st;