Reject r-l/r updates from non-default branches

Author: tgross35

crates/josh-sync/src/sync.rs

@@ -5,6 +5,7 @@ use std::{env, fs, process, thread};
 
 const JOSH_PORT: u16 = 42042;
 const DEFAULT_PR_BRANCH: &str = "update-builtins";
+const DEFAULT_BRANCH: &str = "master";
 
 pub struct GitSync {
     upstream_repo: String,
@@ -179,6 +180,12 @@ impl GitSync {
             .trim()
             .to_string();
 
+        let current_branch = check_output(["git", "branch", "--show-current"]);
+        assert_eq!(
+            current_branch, DEFAULT_BRANCH,
+            "refusing to push from non-default branch"
+        );
+
         // Make sure josh is running.
         let _josh = Josh::start();