repository: Properly set id for finalized index

As MergeFinalIndex and index uploads can occur concurrently, it is
necessary for MergeFinalIndex to check whether the IDs for an index were
already set before merging it. Otherwise, we'd loose the ID of an index
which is set _after_ uploading it.

internal/repository/testing.go

@@ -132,3 +132,13 @@ func BenchmarkAllVersions(b *testing.B, bench VersionedBenchmark) {
 		})
 	}
 }
+
+func TestMergeIndex(t testing.TB, mi *MasterIndex) ([]*Index, int) {
+	finalIndexes := mi.FinalizeNotFinalIndexes()
+	for _, idx := range finalIndexes {
+		test.OK(t, idx.SetID(restic.NewRandomID()))
+	}
+
+	test.OK(t, mi.MergeFinalIndexes())
+	return finalIndexes, len(mi.idx)
+}