fix(restore): allow incrementalFrom to be 1 in restore API

This is a minor inconvenience while using the incremental restore
API that when incrementalFrom is set to 1, we throw an error back.
The restore API takes a from backup number (incrementalFrom) and a
to backup number (backupNum) and restores all the backups including
both the ends, i.e. following set notation all the backups in the
set [incrementalFrom, backupNum] are restored. This should work
fine when incrementalFrom is set to 1 which is a full backup.

systest/integration2/incremental_restore_test.go

@@ -68,9 +68,6 @@ func TestIncrementalRestore(t *testing.T) {
 		t.Logf("restoring backup #%v\n", i)
 
 		incrFrom := i - 1
-		if i == 2 {
-			incrFrom = 0
-		}
 		require.NoError(t, hc.Restore(c, dgraphtest.DefaultBackupDir, "", incrFrom, i, ""))
 		require.NoError(t, dgraphtest.WaitForRestore(c))
 

worker/online_restore.go

@@ -246,7 +246,7 @@ func handleRestoreProposal(ctx context.Context, req *pb.RestoreRequest, pidx uin
 	}
 
 	if req.IncrementalFrom == 1 {
-		return errors.Errorf("Incremental restore must not include full backup")
+		req.IncrementalFrom = 0
 	}
 
 	// Clean up the cluster if it is a full backup restore.