remote-helper: Account for first push from delegate

An edge case was found where a delegate attempted to merge a patch,
but had not pushed to their default branch yet.
This resulted in the `old` not being defined and the check to revert
and/or merge not happening – so the patch continued to appear as open.

Since `old` only matters for the revert and merge check for the
default branch, resolving the `old` side does its best to find a
commit for the checks.
It does this by falling back to getting the canonical head before the
push, and using that as the old.

CHANGELOG.md

@@ -5,6 +5,16 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+## Fixed Bugs
+
+- Previously, a delegate could push to the default branch, for the first time,
+  while attempting to merge a patch. However, the patch would not be marked as
+  merged. The Git remote helper will now fallback to the canonical default
+  branch, if the default branch of the delegate could not be found, and
+  correctly mark the patch as merged.
+
 ## 1.9.1
 
 ## Improvements

crates/radicle-remote-helper/src/push.rs

@@ -800,8 +800,18 @@ where
 {
     let head = *src;
     let dst = dst.with_namespace(nid.into());
-    // It's ok for the destination reference to be unknown, eg. when pushing a new branch.
-    let old = stored.backend.find_reference(dst.as_str()).ok();
+
+    // In some rare cases, pushing to the default branch is a new action for a delegate.
+    // If this is the case, use the canonical head before the push to determine the old side.
+    //
+    // Note that this is only important for reverting and merging, but must happen before the push.
+    let old = stored
+        .backend
+        .find_reference(dst.as_str())
+        .ok()
+        .map(|old| old.peel_to_commit().map(|c| c.id().into()))
+        .transpose()?
+        .or_else(|| stored.canonical_head().map(|(_, oid)| oid).ok());
 
     push_ref(
         src,
@@ -820,11 +830,10 @@ where
         // If we're pushing to the project's default branch, we want to see if any patches got
         // merged or reverted, and if so, update the patch COB.
         if &*dst.strip_namespace() == master {
-            let old = old.peel_to_commit()?.id();
             // Only delegates affect the merge state of the COB.
             if stored.delegates()?.contains(&nid.into()) {
-                patch_revert_all(old.into(), head, &stored.backend, &mut patches)?;
-                patch_merge_all(old.into(), head, working, &mut patches, signer)?;
+                patch_revert_all(old, head, &stored.backend, &mut patches)?;
+                patch_merge_all(old, head, working, &mut patches, signer)?;
             }
         }
     }