upgrade git2 to v20 (#4961)

This should bring support for negative refspecs.

Author: Byron

Cargo.lock

@@ -44,7 +44,7 @@ resolver = "2"
 bstr = "1.11.1"
 # Add the `tracing` or `tracing-detail` features to see more of gitoxide in the logs. Useful to see which programs it invokes.
 gix = { git = "https://github.com/GitoxideLabs/gitoxide", rev = "faa0cdeb35a8135ff9513a1c9884126f6b080f4a", default-features = false, features = [] }
-git2 = { version = "0.19.0", features = [
+git2 = { version = "0.20.0", features = [
     "vendored-openssl",
     "vendored-libgit2",
 ] }