patch 8.1.1327: unnecessary scroll after horizontal split

Problem:    Unnecessary scroll after horizontal split.
Solution:   Don't adjust to fraction if all the text fits in the window.
            (Martin Kunev, closes #4367)
diff --git a/src/window.c b/src/window.c
index f8df1cc..f031989 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5827,9 +5827,13 @@
     int		sline, line_size;
     int		height = wp->w_height;
 
-    // Don't change w_topline when height is zero.  Don't set w_topline when
-    // 'scrollbind' is set and this isn't the current window.
-    if (height > 0 && (!wp->w_p_scb || wp == curwin))
+    // Don't change w_topline in any of these cases:
+    // - window height is 0
+    // - 'scrollbind' is set and this isn't the current window
+    // - window height is sufficient to display the whole buffer
+    if (height > 0
+        && (!wp->w_p_scb || wp == curwin)
+        && (height < wp->w_buffer->b_ml.ml_line_count))
     {
 	/*
 	 * Find a value for w_topline that shows the cursor at the same