Avoid creating temporary store object for git over the wire

Instead, serialize as NAR and send that over, then rehash sever side.
This is alorithmically simpler, but comes at the cost of a newer
parameter to `Store::addToStoreFromDump`.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

src/libstore/store-api.hh

@@ -466,14 +466,23 @@ public:
      * in `dump`, which is either a NAR serialisation (if recursive ==
      * true) or simply the contents of a regular file (if recursive ==
      * false).
-     * `dump` may be drained
      *
-     * \todo remove?
+     * `dump` may be drained.
+     *
+     * @param dumpMethod What serialisation format is `dump`, i.e. how
+     * to deserialize it. Must either match hashMethod or be
+     * `FileSerialisationMethod::Recursive`.
+     *
+     * @param hashMethod How content addressing? Need not match be the
+     * same as `dumpMethod`.
+     *
+     * @todo remove?
      */
     virtual StorePath addToStoreFromDump(
         Source & dump,
         std::string_view name,
-        ContentAddressMethod method = FileIngestionMethod::Recursive,
+        FileSerialisationMethod dumpMethod = FileSerialisationMethod::Recursive,
+        ContentAddressMethod hashMethod = FileIngestionMethod::Recursive,
         HashAlgorithm hashAlgo = HashAlgorithm::SHA256,
         const StorePathSet & references = StorePathSet(),
         RepairFlag repair = NoRepair) = 0;
@@ -772,7 +781,7 @@ protected:
      * Helper for methods that are not unsupported: this is used for
      * default definitions for virtual methods that are meant to be overriden.
      *
-     * \todo Using this should be a last resort. It is better to make
+     * @todo Using this should be a last resort. It is better to make
      * the method "virtual pure" and/or move it to a subclass.
      */
     [[noreturn]] void unsupported(const std::string & op)