merge: add non-optional Merge accessor to the zeroth value

We have a few callers which just need to obtain an object common among all
the merge values. Let's add a non-failing accessor for that purpose.

lib/src/merge.rs

@@ -170,6 +170,11 @@ impl<T> Merge<T> {
         &self.adds
     }
 
+    /// Returns the zeroth added value, which is guaranteed to exist.
+    pub fn first(&self) -> &T {
+        &self.adds[0]
+    }
+
     /// Returns the `index`-th removed value, which is considered belonging to
     /// the `index`-th diff pair.
     pub fn get_remove(&self, index: usize) -> Option<&T> {

lib/src/merged_tree.rs

@@ -156,7 +156,7 @@ impl MergedTree {
     pub fn dir(&self) -> &RepoPath {
         match self {
             MergedTree::Legacy(tree) => tree.dir(),
-            MergedTree::Merge(conflict) => conflict.adds()[0].dir(),
+            MergedTree::Merge(conflict) => conflict.first().dir(),
         }
     }
 
@@ -164,7 +164,7 @@ impl MergedTree {
     pub fn store(&self) -> &Arc<Store> {
         match self {
             MergedTree::Legacy(tree) => tree.store(),
-            MergedTree::Merge(trees) => trees.adds()[0].store(),
+            MergedTree::Merge(trees) => trees.first().store(),
         }
     }
 
@@ -448,7 +448,7 @@ fn merge_trees(merge: &Merge<Tree>) -> Result<Merge<Tree>, TreeMergeError> {
         return Ok(Merge::resolved(tree.clone()));
     }
 
-    let base_tree = &merge.adds()[0];
+    let base_tree = merge.first();
     let store = base_tree.store();
     let dir = base_tree.dir();
     // Keep resolved entries in `new_tree` and conflicted entries in `conflicts` to