patch 8.1.0974: cannot switch from terminal window to previous tabpage

Problem:    Cannot switch from terminal window to previous tabpage.
Solution:   Make CTRL-W gT move to previous tabpage.
diff --git a/src/window.c b/src/window.c
index 83e3e9f..074c731 100644
--- a/src/window.c
+++ b/src/window.c
@@ -87,10 +87,7 @@
 #endif
     char_u	cbuf[40];
 
-    if (Prenum == 0)
-	Prenum1 = 1;
-    else
-	Prenum1 = Prenum;
+    Prenum1 = Prenum == 0 ? 1 : Prenum;
 
 #ifdef FEAT_CMDWIN
 # define CHECK_CMDWIN \
@@ -588,6 +585,10 @@
 			goto_tabpage((int)Prenum);
 			break;
 
+		    case 'T':	    // CTRL-W gT: go to previous tab page
+			goto_tabpage(-(int)Prenum1);
+			break;
+
 		    default:
 			beep_flush();
 			break;