Add slot branch_name to S4 class git_branch
Add function to check if branch is pointed at by HEAD.

* src/git2r_branch.c/h (git2r_branch_is_head): Add function

* src/git2r.c: Add git2r_branch_is_head to callMethods

* src/git2r_error.c (git2r_check_branch_arg): Check branch name