patch 8.1.0994: relative cursor position is not calculated correctly

Problem:    Relative cursor position is not calculated correctly.
Solution:   Always set topline, also when window is one line only.
            (Robert Webb) Add more info to getwininfo() for testing.

runtime/doc/eval.txt

@@ -5228,6 +5228,7 @@ getwininfo([{winid}])					*getwininfo()*
 		tab pages is returned.
 
 		Each List item is a Dictionary with the following entries:
+			botline		last displayed buffer line
 			bufnr		number of buffer in the window
 			height		window height (excluding winbar)
 			loclist		1 if showing a location list
@@ -5237,6 +5238,7 @@ getwininfo([{winid}])					*getwininfo()*
 			terminal	1 if a terminal window
 					{only with the +terminal feature}
 			tabnr		tab page number
+			topline		first displayed buffer line 
 			variables	a reference to the dictionary with
 					window-local variables
 			width		window width