Add a "nix log" command

This replaces "nix-store --read-log". It checks the local store and
any configured substituters for the requested logs.

src/nix/log.cc

@@ -0,0 +1,57 @@
+#include "command.hh"
+#include "common-args.hh"
+#include "installables.hh"
+#include "shared.hh"
+#include "store-api.hh"
+
+using namespace nix;
+
+struct CmdLog : StoreCommand, MixInstallables
+{
+    CmdLog()
+    {
+    }
+
+    std::string name() override
+    {
+        return "log";
+    }
+
+    std::string description() override
+    {
+        return "show the build log of the specified packages or paths";
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto elems = evalInstallables(store);
+
+        PathSet paths;
+
+        for (auto & elem : elems) {
+            if (elem.isDrv)
+                paths.insert(elem.drvPath);
+            else
+                paths.insert(elem.outPaths.begin(), elem.outPaths.end());
+        }
+
+        auto subs = getDefaultSubstituters();
+
+        subs.push_front(store);
+
+        for (auto & path : paths) {
+            bool found = false;
+            for (auto & sub : subs) {
+                auto log = sub->getBuildLog(path);
+                if (!log) continue;
+                std::cout << *log;
+                found = true;
+                break;
+            }
+            if (!found)
+                throw Error("build log of path ‘%s’ is not available", path);
+        }
+    }
+};
+
+static RegisterCommand r1(make_ref<CmdLog>());