Update git2 requirement from 0.20 to 0.21 #160

Job	Run time
Clippy	1m 40s
Test	1m 33s
Format	7s
Security	3m 18s
Build (x86_64-unknown-linux-gnu)	1m 46s
Build (x86_64-pc-windows-msvc)	6m 18s
Build (aarch64-unknown-linux-gnu)	2m 17s
Build (x86_64-apple-darwin)	1m 45s
Release	-1s
	18m 43s

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