Code review src/git2r_stash.c

* Fix missing doxygen documentation.

* Change to have consistent style of argument checking

* src/git2r_arg.c/h (git2r_arg_check_integer_gte_zero): Add function
  to check integer argument and that arg is greater than or equal to
  0.

* src/git2r_error.c/h: Add error message git2r_err_integer_gte_zero_arg