Disallow git dependencies that are not allowed

And ignore git dependencies that are unmatched, since we sometimes use
the crates.io version.

deny.toml

@@ -23,9 +23,11 @@ allow = [
 ]
 
 [sources]
-required-git-spec="rev"
+unknown-git = "deny"
+required-git-spec = "rev"
 allow-git = [
     "https://github.com/ruma/ruma",
     "https://github.com/matrix-org/matrix-rust-sdk",
     "https://gitlab.gnome.org/GNOME/glycin",
 ]
+unused-allowed-source = "allow"