As discussed in the initial fuzzing integration PR[^1], `fuzz_tree.py`'s
implementation was not ideal in terms of coverage and its reading/writing to
hard-coded paths inside `/tmp` was problematic as (among other concerns), it
causes intermittent crashes on ClusterFuzz[^2] when multiple workers execute
the test at the same time on the same machine.

The changes here replace `fuzz_tree.py` completely with a completely new
`fuzz_repo.py` fuzz target which:

- Uses `tempfile.TemporaryDirectory()` to safely manage tmpdir creation and
  tear down, including during multi-worker execution runs.
- Retains the same feature coverage as `fuzz_tree.py`, but it also adds
  considerably more from much smaller data inputs and with less memory consumed
  (and it doesn't even have a seed corpus or target specific dictionary yet.)
- Can likely be improved further in the future by exercising additional features
  of `Repo` to the harness.

Because `fuzz_tree.py` was removed and `fuzz_repo.py` was not derived from it,
the Apache License call outs in the docs were also updated as they only apply to
the singe `fuzz_config.py` file now.

[^1]: gitpython-developers#1901 (comment)
[^2]: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=68355