add note about use of SystemReader, checking that system gitconfig is read
adjust diff stdout
fix bugs with log, selectCommit, add initRepo

* wrap log in try block for new repositories, since no log
* check if revwalker is empty before looping, skip if no log
* add initRepo callback
* check if last commit, there is no diff, check if diff_file empty before looping
skip if last commit, set diff_txt to empty