Gisty.Comments should not implement dummy data

[KEINOS]

Currently Gisty.Comments returns a dummy data if the requested gist ID is equal to gist.DummyID.

go-gisty/gisty/cmd_comments.go

Lines 72 to 80 in f535298

	func (g *Gisty) Comments(gistID string) ([]Comment, error) {
	// Return dummy data if the gist ID is the dummy ID to avoid unwanted request
	// in the example of the test.
	if gistID == DummyID {
	return []Comment{DummyComment}, nil
	}
	return g.comments(gistID, g.AltFunctions.Comments)
	}

But it should use the Gisty.AltFunctions to mock the behavior like the others.

go-gisty/gisty/example_test.go

Lines 84 to 92 in f535298

	obj.AltFunctions.List = func(*list.ListOptions) error {
	// Mock the GitHub API response.
	fmt.Fprint(
	obj.Stdout,
	"d5b9800c636dd78defa4f15894d54d29 Title of gist item2 6 files secret 2022-04-16T06:08:46Z",
	)
	return nil
	}

[KEINOS]

The current behavior of Gisty.Comments is due to simplify the use in the examples.
Writing a mock in the example is bit redundant.