merge: add as_normal(), taken from RefTarget

The `RefTarget::as_normal()` function is not specific to `RefTarget`,
and I plan to use it from `local_working_copy`.

lib/src/merge.rs

@@ -402,6 +402,11 @@ impl<T> Merge<Option<T>> {
         !self.is_absent()
     }
 
+    /// Returns the value if this is present and non-conflicting.
+    pub fn as_normal(&self) -> Option<&T> {
+        self.as_resolved()?.as_ref()
+    }
+
     /// Creates lists of `removes` and `adds` from a `Merge` by dropping
     /// `None` values. Note that the conversion is lossy: the order of `None`
     /// values is not preserved when converting back to a `Merge`.

lib/src/op_store.rs

@@ -115,7 +115,7 @@ impl RefTarget {
 
     /// Returns id if this target is non-conflicting and points to a commit.
     pub fn as_normal(&self) -> Option<&CommitId> {
-        self.as_resolved()?.as_ref()
+        self.merge.as_normal()
     }
 
     /// Returns true if this target points to no commit.