repo: inline single-caller, and surprising, Commit::is_empty()

I would expect `Commit::is_empty()` to check if the commit is empty in
our usual sense, i.e. that there are no changes compared to the
auto-merged parents. However, it would return `false` for any merge
commit (and for the root commit). Since we only use it in one place,
let's inline it there. The use there does seem reasonable, because
it's about abandoning an "uninteresting" working-copy commit.

lib/src/commit.rs

@@ -117,12 +117,6 @@ impl Commit {
         &self.data
     }
 
-    pub fn is_empty(&self) -> bool {
-        let parents = self.parents();
-        // TODO: Perhaps the root commit should also be considered empty.
-        parents.len() == 1 && parents[0].tree_id() == self.tree_id()
-    }
-
     pub fn description(&self) -> &str {
         &self.data.description
     }

lib/src/repo.rs

@@ -835,7 +835,8 @@ impl MutableRepo {
                 .store()
                 .get_commit(&wc_commit_id)
                 .map_err(EditCommitError::WorkingCopyCommitNotFound)?;
-            if wc_commit.is_empty()
+            if wc_commit.parent_ids().len() == 1
+                && wc_commit.parents()[0].tree_id() == wc_commit.tree_id()
                 && wc_commit.description().is_empty()
                 && self.view().heads().contains(wc_commit.id())
             {