fix(api)!: correctly deal with number before :tab

Now nvim_parse_cmd and nvim_create_user_command use a "tab" value which
is the same as the number passed before :tab modifier instead of the
number plus 1, and "tab" value is -1 if :tab modifier is not used.

runtime/doc/api.txt

@@ -1846,7 +1846,7 @@ nvim_parse_cmd({str}, {opts})                               *nvim_parse_cmd()*
           • keeppatterns: (boolean) |:keeppatterns|.
           • lockmarks: (boolean) |:lockmarks|.
           • noswapfile: (boolean) |:noswapfile|.
-          • tab: (integer) |:tab|.
+          • tab: (integer) |:tab|. -1 when omitted.
           • verbose: (integer) |:verbose|. -1 when omitted.
           • vertical: (boolean) |:vertical|.
           • split: (string) Split modifier string, is an empty string when

runtime/lua/man.lua

@@ -671,7 +671,7 @@ function M.open_page(count, smods, args)
   local target = ('%s(%s)'):format(name, sect)
 
   local ok, ret = pcall(function()
-    if not smods.tab and find_man() then
+    if smods.tab == -1 and find_man() then
       vim.cmd.tag({ target, mods = { silent = true, keepalt = true } })
     else
       smods.silent = true