radicle/sigrefs: Find first non-replayed commit

The scenario A₁ ← B₁ ← A₂ ← B₂ would previously have resulted in A₁
being loaded. This is undesriable, since the latest non-replayed commit
is B₁.

The information regarding the order of duplicates, which is preserved by
`seen`, can be used to recover the first commit.

CHANGELOG.md

@@ -5,6 +5,14 @@ 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
+
+- Fix the signed references reading process by correctly choosing the first,
+  non-replayed commit. This only occurs if duplicate signatures are found and
+  the process needs to find the first legitimate commit of the namespace.
+
 ## 1.7.1
 
 ## Fixed Bugs