Update git2 to 0.20.4

Author: lovasoa

Cargo.lock

@@ -164,7 +164,7 @@ webpki-roots = { version = "1.0.0", optional = true }
 whoami = { version = "1.2.1", optional = true }
 stringprep = "0.1.2"
 bstr = { version = "1.6.0", default-features = false, features = ["std"], optional = true }
-git2 = { version = "0.19", default-features = false, optional = true }
+git2 = { version = "0.20.4", default-features = false, optional = true }
 hashlink = "0.11.0"
 # NOTE: *must* remain below 1.7.0 to allow users to avoid the `ahash` cyclic dependency problem by pinning the version
 # https://github.com/tkaitchuck/aHash/issues/95#issuecomment-874150078