cli: Fix rad patch show with merged patches

We were not correctly calculating the base commit. Simply
using the latest revision base does the job for now.

radicle-cli/src/commands/patch/show.rs

@@ -30,9 +30,7 @@ fn show_patch_diff(patch: &patch::Patch, storage: &Repository) -> anyhow::Result
 }
 
 fn patch_commits(patch: &patch::Patch, stored: &Repository) -> anyhow::Result<Vec<term::Line>> {
-    let target_head = patch_merge_target_oid(patch.target(), stored)?;
-    let base_oid = stored.raw().merge_base(target_head, **patch.head())?;
-    let range = format!("{}..{}", base_oid, patch.head());
+    let range = format!("{}..{}", patch.base(), patch.head());
 
     let mut revwalk = stored.revwalk(*patch.head())?;
     let mut lines = Vec::new();

radicle/src/cob/patch.rs

@@ -262,6 +262,16 @@ impl Patch {
             .oid
     }
 
+    /// Get the commit of the target branch on which this patch is based.
+    /// This can change via a patch update.
+    pub fn base(&self) -> &git::Oid {
+        &self
+            .latest()
+            .map(|(_, r)| r)
+            .expect("Patch::base: at least one revision is present")
+            .base
+    }
+
     /// Index of latest revision in the revisions list.
     pub fn version(&self) -> RevisionIx {
         self.revisions