cargo: enable vendored-libgit2, document how to properly dynamically link libgit2 #12827

Job	Run time
build (ubuntu-latest)	4m 23s
build (macos-14)	3m 48s
Build jj-lib without Git support	48s
build (windows-latest)	10m 7s
cargo-deny (advisories)	23s
Check protos	34s
build (ubuntu-latest, --all-features)	5m 31s
cargo-deny (bans licenses sources)	23s
Clippy check	2m 39s
Check that MkDocs can build the docs	23s
Check that MkDocs can build the docs with Poetry 1.8	21s
Check formatting	18s
	29m 38s

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