commit: return nils if the commit isn't signed

Unsigned commits aren't exceptional, so just return normally and
indicate there's nothing there.

Author: carlosmn

ext/rugged/rugged_commit.c

@@ -647,10 +647,14 @@ static VALUE rb_git_commit_extract_signature(int argc, VALUE *argv, VALUE self)
 		git_buf_free(&signed_data);
 	}
 
-	rugged_exception_check(error);
+	if (error == GIT_ENOTFOUND) {
+		ret_arr = rb_ary_new3(2, Qnil, Qnil);
+	} else {
+		rugged_exception_check(error);
 
-	ret_arr = rb_ary_new3(2, rb_str_new(signature.ptr, signature.size),
-			         rb_str_new(signed_data.ptr, signed_data.size));
+		ret_arr = rb_ary_new3(2, rb_str_new(signature.ptr, signature.size),
+				      rb_str_new(signed_data.ptr, signed_data.size));
+	}
 
 	git_buf_free(&signature);
 	git_buf_free(&signed_data);

test/commit_test.rb

@@ -271,6 +271,8 @@ def test_extract_signature
     signature, signed_data = Rugged::Commit.extract_signature(@repo, commit_sha)
     assert_equal exp_signature, signature
     assert_equal exp_signed_data, signed_data
+
+    assert_equal [nil, nil], Rugged::Commit.extract_signature(@repo, "8496071c1b46c854b31185ea97743be6a8774479")
   end
 end