fix: reverse tab navigation

Reverse tab navigation seems to be broken from years, instead of going
back to previous field it jumps to primary button.

Root cause: when all fields are exhausted it's suppose to go to primary
button, however that's not the case for shift-tab.

frappe/public/js/frappe/form/layout.js

@@ -408,7 +408,8 @@ frappe.ui.form.Layout = class Layout {
 					// next row
 					grid_row.grid.grid_rows[grid_row.doc.idx].toggle_view(true);
 				}
-			} else {
+			} else if (!shift) {
+				// End of tab navigation
 				$(this.primary_button).focus();
 			}
 		}