refactor method names/code

* General argument validation for the two methods.
* Rename methods:
    git_path       => _abs2git
    git_files      => abs2git
    absolute_path  => _git2abs
    absolute_files => git2abs
add rel2git method
refactor so that config file is not loaded until run time
Fix: Do not treat changed_files_from_git as relative paths.
rename config key filter_inc_regex => module_regex
Limit changed files to those that match test_regex or module_regex.
Deduplicate git_files independent of --git option.
Catch case where there is no coverage for changes.
make use of rel2git
remove commented out code
move format_duration into TestTracker
show duration for tests