Update git2 to 0.12

Author: JohnTitor

Cargo.toml

@@ -37,7 +37,7 @@ clippy_lints = { version = "0.0.212", path = "clippy_lints" }
 regex = "1"
 semver = "0.9"
 rustc_tools_util = { version = "0.2.0", path = "rustc_tools_util"}
-git2 = { version = "0.11", optional = true }
+git2 = { version = "0.12", optional = true }
 tempfile = { version = "3.1.0", optional = true }
 lazy_static = "1.0"