patch 9.0.1670: resetting local option to global value is inconsistent

Problem:    Resetting local option to global value is inconsistent.
Solution:   Handle "<" specifically for 'scrolloff' and 'sidescrolloff'.
            (closes #12594)

Author: brammool

src/option.c

@@ -2135,10 +2135,14 @@ do_set_option_numeric(
 	    ((flags & P_VI_DEF) || cp_val) ? VI_DEFAULT : VIM_DEFAULT];
     else if (nextchar == '<')
     {
-	// For 'undolevels' NO_LOCAL_UNDOLEVEL means to
-	// use the global value.
 	if ((long *)varp == &curbuf->b_p_ul && opt_flags == OPT_LOCAL)
+	    // for 'undolevels' NO_LOCAL_UNDOLEVEL means using the global value
 	    value = NO_LOCAL_UNDOLEVEL;
+	else if (opt_flags == OPT_LOCAL
+		    && ((long *)varp == &curwin->w_p_siso
+		     || (long *)varp == &curwin->w_p_so))
+	    // for 'scrolloff'/'sidescrolloff' -1 means using the global value
+	    value = -1;
 	else
 	    value = *(long *)get_varp_scope(&(options[opt_idx]), OPT_GLOBAL);
     }

src/testdir/test_options.vim

@@ -954,25 +954,33 @@ func Test_local_scrolloff()
   wincmd w
   call assert_equal(5, &so)
   wincmd w
+  call assert_equal(3, &so)
   setlocal so<
   call assert_equal(5, &so)
+  setglob so=8
+  call assert_equal(8, &so)
+  call assert_equal(-1, &l:so)
   setlocal so=0
   call assert_equal(0, &so)
   setlocal so=-1
-  call assert_equal(5, &so)
+  call assert_equal(8, &so)
 
   call assert_equal(7, &siso)
   setlocal siso=3
   call assert_equal(3, &siso)
   wincmd w
   call assert_equal(7, &siso)
   wincmd w
+  call assert_equal(3, &siso)
   setlocal siso<
   call assert_equal(7, &siso)
+  setglob siso=4
+  call assert_equal(4, &siso)
+  call assert_equal(-1, &l:siso)
   setlocal siso=0
   call assert_equal(0, &siso)
   setlocal siso=-1
-  call assert_equal(7, &siso)
+  call assert_equal(4, &siso)
 
   close
   set so&

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1670,
 /**/
     1669,
 /**/