Add gix-commitgraph::File fuzzer #3789

Job	Run time
test	5m 57s
cargo-deny (advisories)	1m 42s
cargo-deny (bans licenses sources)	27s
installation (win-msvc)	4m 40s
installation (win-gnu)	4m 47s
lint	1m 0s
installation (win32-msvc)	4m 45s
installation (win32-gnu)	4m 46s
pure-rust-build	1m 56s
test-32bit (armv7-linux-androideabi)	1m 22s
test-fast (windows-latest)	4m 38s
test-fast (macos-latest)	4m 23s
WebAssembly (wasm32-unknown-unknown)	48s
test-fast (ubuntu-latest)	3m 19s
WebAssembly (wasm32-wasi)	1m 11s
	45m 41s

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