fix false positive inside guards for NestedFunctionCalls readability check

lib/credo/check/readability/nested_function_calls.ex

@@ -68,6 +68,11 @@ defmodule Credo.Check.Readability.NestedFunctionCalls do
     {nil, acc}
   end
 
+  # We don't look into guards
+  defp traverse({expr, _meta, _args}, acc, _issue) when expr in ~w[defguard defguardp when]a do
+    {nil, acc}
+  end
+
   # We don't look into typespec attributes
   defp traverse({:@, _, [{attr_name, _, _args}]}, acc, _issue)
        when attr_name in ~w[callback macrocallback opaque spec type typep]a do

test/credo/check/readability/nested_function_calls_test.exs

@@ -7,7 +7,24 @@
   # cases NOT raising issues
   #
 
-  test "it should NOT report code with no nested function calls" do
+  test "it should NOT report code with nested guard calls" do
+    """
+    defmodule CredoSampleModule do
+      # Note: for some reason test does not fail when using local functions
+      #       that's why I've added "Kernel." prefix for each case
+      defguardp nested_guardp(data) when Kernel.is_atom(hd(hd(data)))
+      defguard nested_guard(data) when nested_guardp(data) or Kernel.is_binary(hd(hd(data)))
+
+      def nested_guard_def(data) when nested_guard_defp(data) or Kernel.is_binary(hd(hd(data)))
+      defp nested_guard_defp(data) when Kernel.is_atom(hd(hd(data)))
+    end
+    """
+    |> to_source_file()
+    |> run_check(@described_check)
+    |> refute_issues()
+  end
+
+  test "it should NOT report code with nested type calls" do
     """
     defmodule CredoSampleModule do
       @callback callback_name :: Keyword.t(Some.remote(some_arg))