Add `Repo` class fuzz target

This test must perform actual disk IO to effectively test all
functionality, so it is somewhat slow compared to other fuzz targets in
this repo. There might be ways to improve this, but as of this commit it
works well enough.

fuzzing/fuzz-targets/fuzz_repo.py

@@ -0,0 +1,61 @@
+import sys
+import os
+import tempfile
+
+import atheris
+
+with atheris.instrument_imports():
+    # We instrument `test_utils` as well, so it doesn't block coverage analysis in Fuzz Introspector:
+    from test_utils import EnhancedFuzzedDataProvider
+    from dulwich.repo import (
+        Repo,
+        InvalidUserIdentity,
+    )
+
+
+def TestOneInput(data):
+    fdp = EnhancedFuzzedDataProvider(data)
+    with tempfile.TemporaryDirectory() as temp_dir:
+        repo = Repo.init(temp_dir)
+        repo.set_description(fdp.ConsumeRandomBytes())
+        repo.get_description()
+
+        # Generate a minimal set of files based on fuzz data to minimize I/O operations.
+        file_paths = [
+            os.path.join(temp_dir, f"File{i}")
+            for i in range(min(3, fdp.ConsumeIntInRange(1, 3)))
+        ]
+        for file_path in file_paths:
+            with open(file_path, "wb") as f:
+                f.write(fdp.ConsumeRandomBytes())
+
+        try:
+            repo.do_commit(
+                message=fdp.ConsumeRandomBytes(),
+                committer=fdp.ConsumeRandomBytes(),
+                author=fdp.ConsumeRandomBytes(),
+                commit_timestamp=fdp.ConsumeRandomInt(),
+                commit_timezone=fdp.ConsumeRandomInt(),
+                author_timestamp=fdp.ConsumeRandomInt(),
+                author_timezone=fdp.ConsumeRandomInt(),
+            )
+        except InvalidUserIdentity:
+            return -1
+
+        for file_path in file_paths:
+            with open(file_path, "wb") as f:
+                f.write(fdp.ConsumeRandomBytes())
+
+        repo.stage(file_paths)
+        repo.do_commit(
+            message=fdp.ConsumeRandomBytes(),
+        )
+
+
+def main():
+    atheris.Setup(sys.argv, TestOneInput)
+    atheris.Fuzz()
+
+
+if __name__ == "__main__":
+    main()