Reference.pm: Updated target() documentation
Repository.xs: Refactor checkout code

Added git_hv_to_checkout_opts function which may be used
by other code paths that need to interpret checkout options
i.e. merges.
Raw.xs: Added git_hv_to_merge_tree_opts function
Repository.xs: Added merge() (WIP)
15-merge.t: Added tests cases for merging (WIP)