Problem description
I have discovered a bug in the diff-cover library. When an untracked binary file is present in the repository, the library fails to read it and throws the following error:

  File "/Users/user/PycharmProjects/some-project/.direnv/python-3.12.3/lib/python3.12/site-packages/diff_cover/diff_reporter.py", line 172, in src_paths_changed
    num_lines = len(file_handle.readlines())
                    ^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen codecs>", line 322, in decode
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xb3 in position 0: invalid start byte

Steps to Reproduce:

1. Create a new repository or use an existing one.
2. Add a binary file to the repository that is not tracked by the version control system (untracked file).
3. Run diff-cover on this repository with --include-untracked flag

Environment:
diff-cover version: 6.5.1, 9.1.0
Python version: 3.12
Operating System: mac os [Sonomna 14.5]

Additional Information:
The problem is likely related to attempting to read all untracked. It might be helpful to use the mechanism from

self._git_diff()

that checks

self._is_path_excluded(src_path) 

and:

if (
   not self._supported_extensions
   or extension in self._supported_extensions
):

This will ensure that only supported file types are read, preventing the UnicodeDecodeError when encountering binary files.