rewrite: avoid accessing MutRepo::view() when rebasing descendants

Since 94e03f5, we lazily filter out non-heads from `View`'s set
of head. Accessing the `MutRepo::view()` triggers that filtering. This
patch makes `DescendantRebaser` not unnecessarily do that. That speeds
up the rebasing of 162 descendants in the git.git repo from ~3.6 s to
~330 ms. Rebasing 1272 descendants takes ~885 ms.

lib/src/repo.rs

@@ -528,6 +528,10 @@ impl MutableRepo {
         )
     }
 
+    pub fn get_checkout(&mut self) -> CommitId {
+        self.view.borrow().checkout().clone()
+    }
+
     pub fn set_checkout(&mut self, id: CommitId) {
         self.view_mut().set_checkout(id);
     }

lib/src/rewrite.rs

@@ -264,18 +264,17 @@ impl<'settings, 'repo> DescendantRebaser<'settings, 'repo> {
     }
 
     fn update_references(&mut self, old_commit_id: CommitId, new_commit_ids: Vec<CommitId>) {
-        if *self.mut_repo.view().checkout() == old_commit_id {
+        if self.mut_repo.get_checkout() == old_commit_id {
             // We arbitrarily pick a new checkout among the candidates.
             let new_commit_id = new_commit_ids[0].clone();
             let new_commit = self.mut_repo.store().get_commit(&new_commit_id).unwrap();
             self.mut_repo.check_out(self.settings, &new_commit);
         }
 
         if let Some(branch_names) = self.branches.get(&old_commit_id) {
-            let view = self.mut_repo.view();
             let mut branch_updates = vec![];
             for branch_name in branch_names {
-                let local_target = view.get_local_branch(branch_name).unwrap();
+                let local_target = self.mut_repo.get_local_branch(branch_name).unwrap();
                 for old_add in local_target.adds() {
                     if old_add == old_commit_id {
                         branch_updates.push((branch_name.clone(), true));