GitHub fetcher: Don't emit treeHash yet

But do accept it if it's there, so we don't choke on future lock files
that do have the treeHash attribute.

src/libfetchers/github.cc

@@ -111,6 +111,7 @@ struct GitArchiveInputScheme : InputScheme
             "narHash",
             "lastModified",
             "host",
+            "treeHash",
         };
     }
 
@@ -268,7 +269,9 @@ struct GitArchiveInputScheme : InputScheme
     {
         auto [input, tarballInfo] = downloadArchive(store, _input);
 
+        #if 0
         input.attrs.insert_or_assign("treeHash", tarballInfo.treeHash.gitRev());
+        #endif
         input.attrs.insert_or_assign("lastModified", uint64_t(tarballInfo.lastModified));
 
         auto accessor = getTarballCache()->getAccessor(tarballInfo.treeHash, false);