Add example to S4 class git_blob show method

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add example to S4 class git_branch show method

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add example to S4 class git_diff show method

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add example to S4 class git_tag show method

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>