Write and use `FETCH_HEAD` (#7) #39

Job	Run time
cargo-deny	1m 40s
Publish Check	21s
Lint	24s
Test (ubuntu-22.04, --features git)	2m 1s
Test (ubuntu-22.04, --features sparse)	1m 15s
Test (ubuntu-22.04, --features local-builder,sparse)	1m 20s
	7m 1s

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