commit_builder: inline mut_repo.write_commit()

As the doc comment says, it's called only from CommitBuilder. Let's clarify
that. I'm also planning to extract a builder that only writes to the store
(without mutably borrowing a mut_repo.) It will help implement description
template.

lib/src/commit_builder.rs

@@ -205,9 +205,8 @@ impl CommitBuilder<'_> {
         // if we're rewriting a signed commit
         self.commit.secure_sig = None;
 
-        let commit = self
-            .mut_repo
-            .write_commit(self.commit, signing_fn.as_deref_mut())?;
+        let commit = store.write_commit(self.commit, signing_fn.as_deref_mut())?;
+        self.mut_repo.add_head(&commit)?;
         if let Some(rewrite_source) = self.rewrite_source {
             if rewrite_source.change_id() == commit.change_id() {
                 self.mut_repo

lib/src/repo.rs

@@ -30,7 +30,7 @@ use tracing::instrument;
 use self::dirty_cell::DirtyCell;
 use crate::backend::{
     Backend, BackendError, BackendInitError, BackendLoadError, BackendResult, ChangeId, CommitId,
-    MergedTreeId, SigningFn,
+    MergedTreeId,
 };
 use crate::commit::{Commit, CommitByCommitterTimestamp};
 use crate::commit_builder::CommitBuilder;
@@ -58,7 +58,7 @@ use crate::store::Store;
 use crate::submodule_store::SubmoduleStore;
 use crate::transaction::Transaction;
 use crate::view::View;
-use crate::{backend, dag_walk, op_store, revset};
+use crate::{dag_walk, op_store, revset};
 
 pub trait Repo {
     fn store(&self) -> &Arc<Store>;
@@ -831,17 +831,6 @@ impl MutableRepo {
         // `self.rewritten_commits`
     }
 
-    /// Only called from [`CommitBuilder::write`]. Use that function instead.
-    pub(crate) fn write_commit(
-        &mut self,
-        commit: backend::Commit,
-        sign_with: Option<&mut SigningFn>,
-    ) -> BackendResult<Commit> {
-        let commit = self.store().write_commit(commit, sign_with)?;
-        self.add_head(&commit)?;
-        Ok(commit)
-    }
-
     /// Record a commit as having been rewritten to another commit in this
     /// transaction.
     ///