patch 8.0.0490: vertical split makes 'winfixwidth' window smaller

Problem:    Splitting a 'winfixwidth' window vertically makes it one column
            smaller. (Dominique Pelle)
Solution:   Add one to the width for the separator.

Author: brammool

src/testdir/test_window_cmd.vim

@@ -294,20 +294,14 @@ func Test_window_width()
   set winfixwidth
   vsplit Xc
   let [ww1, ww2, ww3] = [winwidth(1), winwidth(2), winwidth(3)]
-  " FIXME: commented out: I would expect the width of 2nd window to 
-  " remain 2 but it's actually 1?!
-  "call assert_equal(2, winwidth(2))
+  call assert_equal(2, winwidth(2))
   call assert_inrange(ww3, ww3 + 1, ww1)
   3wincmd >
-  " FIXME: commented out: I would expect the width of 2nd window to 
-  " remain 2 but it's actually 1?!
-  "call assert_equal(2,       winwidth(2))
+  call assert_equal(2,       winwidth(2))
   call assert_equal(ww1 + 3, winwidth(1))
   call assert_equal(ww3 - 3, winwidth(3))
   wincmd =
-  " FIXME: commented out: I would expect the width of 2nd window to 
-  " remain 2 but it's actually 1?!
-  "call assert_equal(2,   winwidth(2))
+  call assert_equal(2,   winwidth(2))
   call assert_equal(ww1, winwidth(1))
   call assert_equal(ww3, winwidth(3))
 

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    490,
 /**/
     489,
 /**/

src/window.c

@@ -870,9 +870,9 @@ win_split_ins(
 
 	/* We don't like to take lines for the new window from a
 	 * 'winfixwidth' window.  Take them from a window to the left or right
-	 * instead, if possible. */
+	 * instead, if possible. Add one for the separator. */
 	if (oldwin->w_p_wfw)
-	    win_setwidth_win(oldwin->w_width + new_size, oldwin);
+	    win_setwidth_win(oldwin->w_width + new_size + 1, oldwin);
 
 	/* Only make all windows the same width if one of them (except oldwin)
 	 * is wider than one of the split windows. */