repo: Split packers for tree and data

The code now bundles tree blobs and data blobs into different pack
files, so we'll end up with pack files that either only contain data or
trees. This is in preparation to adding a cache (#1040), because
tree-only pack files can easily be cached later on.

internal/repository/packer_manager_test.go

@@ -72,7 +72,7 @@ func fillPacks(t testing.TB, rnd *randReader, be Saver, pm *packerManager, buf [
 		l := rnd.rand.Intn(1 << 20)
 		seed := rnd.rand.Int63()
 
-		packer, err := pm.findPacker(uint(l))
+		packer, err := pm.findPacker()
 		if err != nil {
 			t.Fatal(err)
 		}