Replace the Suboptimal `fuzz_tree.py` Harness With a Better Alternative

[DaveLak]

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.

Here are some very rough stats to give a very rough idea of the difference:

Metric	fuzz_repo.py (my local testing)	fuzz_tree.py (most recent successful CF run)
Coverage	789	163
Features	1023	269
Corpus	19 entries / 57 bytes	30 entries / 1872 bytes

Note on License

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.

[DaveLak]

CC @EliahKagan

[DaveLak]

and FWIW, if removing fuzz_tree is not desired for any reason, I did test out tempfile.TemporaryDirectory() there and it works well enough to address the primary concerns raised. My fork has the commit with it: https://github.com/DaveLak/GitPython/commit/3a049599924d081fbeb1a8662ee6d2a2af4af80c