vim-patch:9.1.1961: :0tab behaves like :tab for :stag when 'swb' contains "newtab"

Problem:  :0tab behaves like :tab for :stag when 'switchbuf' contains
          "newtab" (after 9.1.1949).
Solution: Don't override cmod_tab if it's already non-zero (zeertzjq).

closes: vim/vim#18859

2b3566d89e

src/nvim/tag.c

@@ -2869,7 +2869,7 @@ static int jumpto_tag(const char *lbuf_arg, int forceit, bool keep_help)
       cmdmod.cmod_split |= WSP_VERT;
     }
 
-    if (swb_flags & kOptSwbFlagNewtab) {
+    if ((swb_flags & kOptSwbFlagNewtab) && cmdmod.cmod_tab == 0) {
       // If 'switchbuf' contains 'newtab', then use a new tabpage
       cmdmod.cmod_tab = tabpage_index(curtab) + 1;
     }

test/old/testdir/test_tagjump.vim

@@ -163,7 +163,12 @@ func Test_tagjump_switchbuf()
   call assert_equal(2, tabpagenr('$'))
   call assert_equal(2, tabpagenr())
   call assert_equal(2, line('.'))
+  0tab stag third
+  call assert_equal(3, tabpagenr('$'))
+  call assert_equal(1, tabpagenr())
+  call assert_equal(3, line('.'))
 
+  tabclose!
   tabclose!
   enew | only
   set tags&