agent2: Initial infra for checkpoints and message editing (#36120)

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/git/src/git.rs

@@ -119,6 +119,13 @@ impl Oid {
         Ok(Self(oid))
     }
 
+    #[cfg(any(test, feature = "test-support"))]
+    pub fn random(rng: &mut impl rand::Rng) -> Self {
+        let mut bytes = [0; 20];
+        rng.fill(&mut bytes);
+        Self::from_bytes(&bytes).unwrap()
+    }
+
     pub fn as_bytes(&self) -> &[u8] {
         self.0.as_bytes()
     }