Only use relative paths with function is_ignored()

[ericwb]

The is_ignored() function expects paths relative to the starting path given when initializing the class. It raises a traceback if given an absolute path.

Traceback (most recent call last):
  File "/Users/ericwb/workspace/precli/.tox/py312/bin/precli", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/ericwb/workspace/precli/precli/cli/main.py", line 301, in main
    file_list = discover_files(args.targets, args.recursive)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ericwb/workspace/precli/precli/cli/main.py", line 177, in discover_files
    if not preignore_mgr.is_ignored(path):
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ericwb/workspace/precli/.tox/py312/lib/python3.12/site-packages/ignorelib.py", line 401, in is_ignored
    matches = list(self._find_matching(path))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ericwb/workspace/precli/.tox/py312/lib/python3.12/site-packages/ignorelib.py", line 368, in _find_matching
    raise ValueError('%s is an absolute path' % path)
ValueError: /var/folders/m3/_cy_9dfx73107r3w_zhnsd700000gn/T/tmp_100ms8v/securesauce-precli-2ef4388/LICENSE is an absolute path