Prefix C object names with git2r_ in src/git2r_tree.c/h
Prefix git2r C functions with git2r_modulename_

* Changed function names to have similar naming scheme as libgit2 with
  function names also prefixed with
  modulename. git2r_modulename_functionname() (e.g. git2r_repository_open())
Roxygenized with roxygen2_4.0.1