checkout: silence diffs when checking validity of a commit

Otherwise, when people submit treewide PRs to nixpkgs, the logs get
cluttered with thousands of useless diffs.

ofborg/src/checkout.rs

@@ -117,6 +117,7 @@ impl CachedProjectCo {
         let result = Command::new("git")
             .arg("--no-pager")
             .arg("show")
+            .arg("--no-patch")
             .arg(commit)
             .current_dir(self.clone_to())
             .status()