Fix method build_structure of class SimpleTreeMapBuilder to use parent_dir parameter instead of global rootdir variable
    Modify treemap.py
refactor treemap.py, use absolute path as rootdir
optimize treemap.py
    Add class MyGit, which caches all commits, and changed files per commit
    Add method get_log to class MyGit to get commits per file and author
    Modify methods __get_last_commits of class GitFile, and GitFileByAuthor to use method MyGit.get_log
optimize treemap.py, and fix bug that changed lines are shown per commit instead of per file per commit
    Add method get_changed_lines to class MyGit to get changed lines per file
    Replace method _get_changed_lines_of_commit of class GitFile with method MyGit.get_changed_lines