Add gix-commitgraph::File fuzzer #3785

Job	Run time
pure-rust-build	1m 54s
installation (win-msvc)	6m 19s
lint	3m 40s
test	21m 3s
installation (win-gnu)	6m 49s
test-fast (windows-latest)	14m 33s
WebAssembly (wasm32-unknown-unknown)	1m 9s
installation (win32-msvc)	7m 13s
test-fast (macos-latest)	6m 11s
WebAssembly (wasm32-wasi)	1m 6s
installation (win32-gnu)	6m 38s
test-fast (ubuntu-latest)	5m 13s
cargo-deny (advisories)	28s
cargo-deny (bans licenses sources)	30s
test-32bit (armv7-linux-androideabi)	1m 46s
	1h 24m 32s

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