repo: do not discard old working-copy commit if it's pointed by local branch

It would be confusing if a branch moved backwards by checking out unrelated
commit.

#1854

lib/src/repo.rs

@@ -857,6 +857,13 @@ impl MutableRepo {
         workspace_id: WorkspaceId,
         commit: &Commit,
     ) -> Result<(), EditCommitError> {
+        fn local_branch_target_ids(view: &View) -> impl Iterator<Item = &CommitId> {
+            view.branches()
+                .values()
+                .filter_map(|branch_target| branch_target.local_target.as_ref())
+                .flat_map(|target| target.adds())
+        }
+
         let maybe_wc_commit_id = self
             .view
             .with_ref(|v| v.get_wc_commit_id(&workspace_id).cloned());
@@ -865,8 +872,14 @@ impl MutableRepo {
                 .store()
                 .get_commit(&wc_commit_id)
                 .map_err(EditCommitError::WorkingCopyCommitNotFound)?;
-            if wc_commit.is_discardable() && self.view().heads().contains(wc_commit.id()) {
-                // Abandon the working-copy commit we're leaving if it's empty and a head commit
+            if wc_commit.is_discardable()
+                && self
+                    .view
+                    .with_ref(|v| local_branch_target_ids(v).all(|id| id != wc_commit.id()))
+                && self.view().heads().contains(wc_commit.id())
+            {
+                // Abandon the working-copy commit we're leaving if it's empty, not pointed by
+                // local branch, and a head commit.
                 self.record_abandoned_commit(wc_commit_id);
             }
         }

lib/tests/test_mut_repo.rs

@@ -151,6 +151,41 @@ fn test_checkout_previous_empty_with_description(use_git: bool) {
     assert!(mut_repo.view().heads().contains(old_wc_commit.id()));
 }
 
+#[test_case(false ; "local backend")]
+#[test_case(true ; "git backend")]
+fn test_checkout_previous_empty_with_local_branch(use_git: bool) {
+    // Test that MutableRepo::check_out() does not abandon the previous commit if it
+    // is pointed by local branch.
+    let settings = testutils::user_settings();
+    let test_repo = TestRepo::init(use_git);
+    let repo = &test_repo.repo;
+
+    let mut tx = repo.start_transaction(&settings, "test");
+    let mut_repo = tx.mut_repo();
+    let old_wc_commit = mut_repo
+        .new_commit(
+            &settings,
+            vec![repo.store().root_commit_id().clone()],
+            repo.store().empty_tree_id().clone(),
+        )
+        .write()
+        .unwrap();
+    mut_repo.set_local_branch(
+        "b".to_owned(),
+        RefTarget::Normal(old_wc_commit.id().clone()),
+    );
+    let ws_id = WorkspaceId::default();
+    mut_repo.edit(ws_id.clone(), &old_wc_commit).unwrap();
+    let repo = tx.commit();
+
+    let mut tx = repo.start_transaction(&settings, "test");
+    let mut_repo = tx.mut_repo();
+    let new_wc_commit = write_random_commit(mut_repo, &settings);
+    mut_repo.edit(ws_id, &new_wc_commit).unwrap();
+    mut_repo.rebase_descendants(&settings).unwrap();
+    assert!(mut_repo.view().heads().contains(old_wc_commit.id()));
+}
+
 #[test_case(false ; "local backend")]
 #[test_case(true ; "git backend")]
 fn test_checkout_previous_empty_non_head(use_git: bool) {