Add 'nix nar dump-path'

This only differs from 'nix store dump-path' in that the path doesn't
need to be a store path.

src/nix/dump-path.cc

@@ -1,5 +1,6 @@
 #include "command.hh"
 #include "store-api.hh"
+#include "archive.hh"
 
 using namespace nix;
 
@@ -7,7 +8,7 @@ struct CmdDumpPath : StorePathCommand
 {
     std::string description() override
     {
-        return "dump a store path to stdout (in NAR format)";
+        return "serialise a store path to stdout in NAR format";
     }
 
     Examples examples() override
@@ -30,4 +31,43 @@ struct CmdDumpPath : StorePathCommand
     }
 };
 
-static auto rDumpPath = registerCommand<CmdDumpPath>("dump-path");
+
+static auto rDumpPath = registerCommand2<CmdDumpPath>({"store", "dump-path"});
+
+struct CmdDumpPath2 : Command
+{
+    Path path;
+
+    CmdDumpPath2()
+    {
+        expectArgs({
+            .label = "path",
+            .handler = {&path},
+            .completer = completePath
+        });
+    }
+
+    std::string description() override
+    {
+        return "serialise a path to stdout in NAR format";
+    }
+
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To serialise directory 'foo' as a NAR:",
+                "nix nar dump-path ./foo"
+            },
+        };
+    }
+
+    void run() override
+    {
+        FdSink sink(STDOUT_FILENO);
+        dumpPath(path, sink);
+        sink.flush();
+    }
+};
+
+static auto rDumpPath2 = registerCommand2<CmdDumpPath2>({"nar", "dump-path"});

src/nix/main.cc

@@ -125,6 +125,7 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
         {"ls-store", {"store", "ls"}},
         {"cat-nar", {"nar", "cat"}},
         {"cat-store", {"store", "cat"}},
+        {"dump-path", {"store", "dump-path"}},
     };
 
     bool aliasUsed = false;