merge: add Merge::is_absent() and Merge::is_present()

These turned out to only be (very marginally) useful in `RefTarget`
right now, but I plan to add a few more uses elsewhere.

lib/src/merge.rs

@@ -311,6 +311,14 @@ impl<T> Merge<Option<T>> {
         Self::resolved(Some(value))
     }
 
+    pub fn is_absent(&self) -> bool {
+        matches!(self.as_resolved(), Some(None))
+    }
+
+    pub fn is_present(&self) -> bool {
+        !self.is_absent()
+    }
+
     /// 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

@@ -105,13 +105,13 @@ impl RefTarget {
 
     /// Returns true if this target points to no commit.
     pub fn is_absent(&self) -> bool {
-        matches!(self.merge.as_resolved(), Some(None))
+        self.merge.is_absent()
     }
 
     /// Returns true if this target points to any commit. Conflicting target is
     /// always "present" as it should have at least one commit id.
     pub fn is_present(&self) -> bool {
-        !self.is_absent()
+        self.merge.is_present()
     }
 
     /// Whether this target has conflicts.