fix: Pin down git2 dependencies to avoid segfault

See also [1][2]. Fixes #307.

[1] rust-lang/git2-rs#813
[2] rust-lang/cargo#10446

Author: Frederick888

Cargo.lock

@@ -31,7 +31,9 @@ name = "cargo-outdated"
 anyhow = "1.0"
 cargo = "0.60.0"
 env_logger = "0.9.0"
-git2-curl = "0.14.0"
+git2 = "=0.13.23"
+git2-curl = "=0.14.1"
+libgit2-sys = "=0.12.24"
 semver = "1.0.0"
 serde = {version="1.0.114", features = ["derive"]}
 serde_derive = "1.0.114"