patch 8.1.2072: "gk" moves to start of line instead of upwards

Problem:    "gk" moves to start of line instead of upwards.
Solution:   Fix off-by-one error. (Christian Brabandt, closes #4969)
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index c6df183..dcfa929 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -2633,3 +2633,25 @@
   call assert_equal('100 foobar foobar fo', getreg(0))
   bw!
 endfunc
+
+func Test_normal_gk()
+  " needs 80 column new window
+  new
+  vert 80new
+  put =[repeat('x',90)..' {{{1', 'x {{{1']
+  norm! gk
+  " In a 80 column wide terminal the window will be only 78 char
+  " (because Vim will leave space for the other window),
+  " but if the terminal is larger, it will be 80 chars, so verify the
+  " cursor column correctly.
+  call assert_equal(winwidth(0)+1, col('.'))
+  call assert_equal(winwidth(0)+1, virtcol('.'))
+  norm! j
+  call assert_equal(6, col('.'))
+  call assert_equal(6, virtcol('.'))
+  norm! gk
+  call assert_equal(95, col('.'))
+  call assert_equal(95, virtcol('.'))
+  bw!
+  bw!
+endfunc