handle branches via 'git-python' module
[WIP] reimplment tree() function
fixup! [WIP] reimplment tree() function

+ fixed command 'list'
better error message
replace hard coded branch name with ITDB_BRANCH
fixup! [WIP] reimplment tree() function
code cleanup
fixup! [WIP] reimplment tree() function

+ removed tree() function
remove all lib/libgit.py dependencies for command 'init'

+ replaces all libgit function with <git.cmd> executions
add remark for fixes
fix whitespace
save and use Repo().working_dir in ticket object
avoid unnecessary filename() call
replace find_root() with Repo().working_dir
replace find_git_repo() with Repo().git_dir
remove obsolete lib/repo.py
replace cat_file() with Repo.git.cat_file()
replace change_head_branch() with Repo().git.symbolic_ref()
replace has_*_changes() with Repo().is_dirty()

+ removed has_uncommited_changes()
+ removed has_unstaged_changes()
+ removed command_exitcode_only()
replace command_lines() with GitPython module