Refactoring of S4 class git_merge_result

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add merger argument to merge method

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Create and commit merge commit

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

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