patch 9.1.0147: Cannot keep a buffer focused in a window

Problem:  Cannot keep a buffer focused in a window
          (Amit Levy)
Solution: Add the 'winfixbuf' window-local option
          (Colin Kennedy)

fixes:  #6445
closes: #13903

Signed-off-by: Colin Kennedy <colinvfx@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/optwin.vim b/runtime/optwin.vim
index 6e133ce..d3e1605 100644
--- a/runtime/optwin.vim
+++ b/runtime/optwin.vim
@@ -482,6 +482,7 @@
   call <SID>AddOption("statusline", gettext("alternate format to be used for a status line"))
   call <SID>OptionG("stl", &stl)
 endif
+call append("$", "\t" .. s:local_to_window)
 call <SID>AddOption("equalalways", gettext("make all windows the same size when adding/removing windows"))
 call <SID>BinOptionG("ea", &ea)
 call <SID>AddOption("eadirection", gettext("in which direction 'equalalways' works: \"ver\", \"hor\" or \"both\""))
@@ -490,6 +491,8 @@
 call append("$", " \tset wh=" . &wh)
 call <SID>AddOption("winminheight", gettext("minimal number of lines used for any window"))
 call append("$", " \tset wmh=" . &wmh)
+call <SID>AddOption("winfixbuf", gettext("keep window focused on a single buffer"))
+call <SID>OptionG("wfb", &wfb)
 call <SID>AddOption("winfixheight", gettext("keep the height of the window"))
 call append("$", "\t" .. s:local_to_window)
 call <SID>BinOptionL("wfh")