patch 9.1.0294: Text height function does not respect it's argument

Problem:  plines_m_win() does not take into account it's "limit_winheight"
          argument for filler lines below the last line of the buffer.
          (after v9.1.0280)
Solution: Check window height when "limit_winheight" is TRUE.
          (Luuk van Baal)

closes: #14449

Signed-off-by: Luuk van Baal <luukvbaal@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_diffmode.vim

@@ -2025,17 +2025,19 @@ endfunc
   bwipe!
   bwipe!
 endfunc
-
+
 " Ctrl-D reveals filler lines below the last line in the buffer.
 func Test_diff_eob_halfpage()
-  5new
-  call setline(1, ['']->repeat(10) + ['a'])
+  new
   call setline(1, ['']->repeat(10) + ['a'])
   diffthis
+  new
   call setline(1, ['']->repeat(3) + ['a', 'b'])
-  call setline(1, ['']->repeat(3) + ['a', 'b'])
-  diffthis
-  wincmd j
+  diffthis
+  resize 5
+  wincmd j
+  resize 5
+  norm G
   call assert_equal(7, line('w0'))
   exe "norm! \<C-D>"
   call assert_equal(8, line('w0'))