Index.xs: Added has_conflicts() method
Index.xs: Added conflict_cleanup() method
Index.pm: Added documentation for has_conflicts()
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: git_repository_state() does not return an error code, only the state
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)