Add git2-rs repo under automation

[Kobzol]

Repo: https://github.com/rust-lang/git2-rs

Assigned to cargo according to https://hackmd.io/@rust-leadership-council/Bk6ge9Xu6, but not sure about it.

CC @rust-lang/cargo

Extracted from GH:

org = "rust-lang"
name = "git2-rs"
description = "libgit2 bindings for Rust"
bots = []

[access.teams]
security = "pull"

[access.individuals]
joshtriplett = "admin"
Mark-Simulacrum = "admin"
rylev = "admin"
pietroalbini = "admin"
badboy = "admin"
rust-lang-owner = "admin"
jdno = "admin"
alexcrichton = "write"
ehuss = "write"

[[branch-protections]]
pattern = "master"
ci-checks = ["Success gate"]
dismiss-stale-review = false
pr-required = true
review-required = false

[ehuss]

I think this is fine for now. We don't really know what the long-term strategy will be for this library.