Disallow exiting from rename: mode with backspace

After looking at the list of possibilities for `app.ui.cmdPrefix`
by searching through the code, renaming is the only remaining mode
where exiting via backspace doesn't make sense.

If I missed something, please let me know. I can fix it here
or in a subsequent PR, depending on when we find out.

Fixes #1052, #1059

eval.go

@@ -1887,8 +1887,9 @@ func (e *callExpr) eval(app *app, args []string) {
 			switch app.ui.cmdPrefix {
 			case "!", "$", "%", "&":
 				app.ui.cmdPrefix = ":"
-			case ">":
-				// Don't mess with the program waiting for input
+			case ">", "rename: ":
+				// Don't mess with the programs waiting for input
+				// Exiting on backspace is also inconvenient for renames
 			default:
 				normal(app)
 			}