GitArchiveInputScheme: Bring back the narHash attribute

This is needed to verify that the source tree served by GitHub hasn't
changed compared to the lock file. Computing the narHash for a nixpkgs
source tree only takes ~0.6s and it's cached. So the cost is fairly
negligible compared to the download time.

src/libfetchers/input-accessor.hh

@@ -5,6 +5,7 @@
 #include "archive.hh"
 #include "canon-path.hh"
 #include "repair-flag.hh"
+#include "hash.hh"
 
 namespace nix {
 
@@ -57,6 +58,11 @@ struct InputAccessor : public std::enable_shared_from_this<InputAccessor>
         Sink & sink,
         PathFilter & filter = defaultPathFilter);
 
+    Hash hashPath(
+        const CanonPath & path,
+        PathFilter & filter = defaultPathFilter,
+        HashType ht = htSHA256);
+
     StorePath fetchToStore(
         ref<Store> store,
         const CanonPath & path,