Fix top level config options not applied to stdin

spec/cli_spec.lua

@@ -917,6 +917,14 @@ Total: 0 warnings / 0 errors in 1 file
 ]], get_output "spec/samples/compat.lua --config=spec/configs/global_config.luacheckrc")
          end)
 
+         it("uses config when checking stdin", function()
+            assert.equal([[
+Checking stdin                                    OK
+
+Total: 0 warnings / 0 errors in 1 file
+]], get_output "- --config=spec/configs/global_config.luacheckrc < spec/samples/compat.lua")
+         end)
+
          it("uses per-file overrides", function()
             assert.equal([[
 Checking spec/samples/bad_code.lua                Failure

src/luacheck/config.lua

@@ -253,15 +253,20 @@ function config.get_top_options(conf)
    return conf.empty and {} or conf.options
 end
 
--- Returns array of options for a path.
-function config.get_options(conf, path)
+-- Returns array of options for a file.
+function config.get_options(conf, file)
    if conf.empty then
       return {}
    end
 
-   path = fs.normalize(fs.join(conf.cur_dir, path))
    local res = {conf.options}
 
+   if type(file) ~= "string" then
+      return res
+   end
+
+   local path = fs.normalize(fs.join(conf.cur_dir, file))
+
    for _, override_path in ipairs(conf.overrides.paths) do
       if fs.is_subpath(override_path, path) then
          table.insert(res, conf.overrides.options[override_path])

src/luacheck/main.lua

@@ -416,12 +416,8 @@ Otherwise, the pattern matches warning code.]])
       local res = {}
 
       for i, file in ipairs(files) do
-         if type(file) == "string" then
-            res[i] = config.get_options(conf, file)
-            table.insert(res[i], cli_opts)
-         else
-            res[i] = cli_opts
-         end
+         res[i] = config.get_options(conf, file)
+         table.insert(res[i], cli_opts)
       end
 
       return res