Accept path argument
output_line: rename argument
Print relative path for filenames
move `output_line()` code to `main()`
Merge branch 'feature/arg_dir' into develop
Make `is_directory` variable meaningful
Merge branch 'feature/refactor' into develop
Print files ordered
Suppress exception if `git ls-tree` fails

The error message "fatal: Not a valid object name HEAD" is
quite useful (you forgot git-flow init!) therefore not suppressed.
Merge branch 'feature/print-order' into develop
Suppress exception when `git rev-parse` fails

If path is not a git repo, it fails with error message.
Merge branch 'feature/path' into develop