bump git2 to 0.13.20 to compile with nightly rustc

git2 0.13.20 has PR rust-lang/git2-rs#712.

Author: martinvonz

Cargo.lock

@@ -23,7 +23,7 @@ test = false
 [build-dependencies]
 # We don't need network functionality. So disable default features which
 # pull it in.
-git2 = { version = "0.13", default-features = false }
+git2 = { version = "0.13.20", default-features = false }
 
 [dependencies]
 anyhow = "1.0"