If gc-keep-derivations is set, only keep the actual deriver

This prevents zillions of derivations from being kept, and fixes an
infinite recursion in the garbage collector (due to an obscure cycle
that can occur with fixed-output derivations).

src/libstore/gc.cc

@@ -473,7 +473,7 @@ bool LocalStore::tryToDelete(GCState & state, const Path & path)
             if (state.gcKeepDerivations && isDerivation(p)) {
                 PathSet outputs = queryDerivationOutputs(p);
                 foreach (PathSet::iterator, i, outputs)
-                    if (isValidPath(*i)) todo.insert(*i);
+                    if (isValidPath(*i) && queryDeriver(*i) == p) todo.insert(*i);
             }
             /* If gc-keep-outputs is set, then don't delete this path
                if there are derivers of this path that are not