Index.xs: Added has_conflicts(), conflict_cleanup() methods
Index.pm: Added documentation for has_conflicts(), conflict_cleanup() methods
Reference.xs: Allow target() to take a new target
Raw.xs: Added git_hv_to_checkout_opts function
Raw.xs: Added git_hv_to_merge_tree_opts function
Repository.xs: Use common checkout options code
Repository.xs: Added checkout_head() method
Repository.xs: Added merge() (WIP)
15-merge.t: Added tests cases for merging (WIP)