This package has DEPEND="dev-libs/libgit2:=" but it's actually not used. libgit2-sys crate is only compatible with a specific range of libgit2 and will build its own version if no compatible version is available. https://github.com/rust-lang/git2-rs/blob/libgit2-sys-0.14.2%2B1.5.1/libgit2-sys/build.rs#L20-L22 I haven't looked into it deeply, but presumably other packages have similar problems (like sys-apps/bat).
