view: unify set/clear_git_head() functions to take Option<RefTarget>

If we migrate RefTarget to new Conflict-based type, it won't store
Conflict<CommitId>, but Conflict<Option<CommitId>>. As the Option will
be internalized, new RefTarget type will also represent an absent target.
The 'target: Option<RefTarget>' argument will be replaced with new RefTarget
type.

I've also renamed the function for consistency with the following changes.
It would be surprising if set_local_branch(name, target) could remove the
branch. I feel the name set_local_branch_target() is less confusing.

lib/src/git.rs

@@ -203,10 +203,10 @@ pub fn import_some_refs(
             let head_commit = store.get_commit(&head_commit_id).unwrap();
             prevent_gc(git_repo, &head_commit_id)?;
             mut_repo.add_head(&head_commit);
-            mut_repo.set_git_head(RefTarget::Normal(head_commit_id));
+            mut_repo.set_git_head_target(Some(RefTarget::Normal(head_commit_id)));
         }
     } else {
-        mut_repo.clear_git_head();
+        mut_repo.set_git_head_target(None);
     }
 
     let mut changed_git_refs = BTreeMap::new();

lib/src/repo.rs

@@ -1023,12 +1023,8 @@ impl MutableRepo {
         self.view.with_ref(|v| v.git_head().cloned())
     }
 
-    pub fn set_git_head(&mut self, target: RefTarget) {
-        self.view_mut().set_git_head(target);
-    }
-
-    pub fn clear_git_head(&mut self) {
-        self.view_mut().clear_git_head();
+    pub fn set_git_head_target(&mut self, target: Option<RefTarget>) {
+        self.view_mut().set_git_head_target(target);
     }
 
     pub fn set_view(&mut self, data: op_store::View) {
@@ -1152,16 +1148,13 @@ impl MutableRepo {
             );
         }
 
-        if let Some(new_git_head) = merge_ref_targets(
+        let new_git_head_target = merge_ref_targets(
             self.index(),
             self.view().git_head(),
             base.git_head(),
             other.git_head(),
-        ) {
-            self.set_git_head(new_git_head);
-        } else {
-            self.clear_git_head();
-        }
+        );
+        self.set_git_head_target(new_git_head_target);
     }
 
     /// Finds and records commits that were rewritten or abandoned between

lib/src/view.rs

@@ -258,12 +258,10 @@ impl View {
         self.data.git_refs.remove(name);
     }
 
-    pub fn set_git_head(&mut self, target: RefTarget) {
-        self.data.git_head = Some(target);
-    }
-
-    pub fn clear_git_head(&mut self) {
-        self.data.git_head = None;
+    /// Sets `HEAD@git` to point to the given target. If the target is absent,
+    /// the reference will be cleared.
+    pub fn set_git_head_target(&mut self, target: Option<RefTarget>) {
+        self.data.git_head = target;
     }
 
     pub fn set_view(&mut self, data: op_store::View) {

lib/tests/test_revset.rs

@@ -649,7 +649,7 @@ fn test_resolve_symbol_git_head() {
     "###);
 
     // With HEAD@git
-    mut_repo.set_git_head(RefTarget::Normal(commit1.id().clone()));
+    mut_repo.set_git_head_target(Some(RefTarget::Normal(commit1.id().clone())));
     insta::assert_debug_snapshot!(
         resolve_symbol(mut_repo, "HEAD", None).unwrap_err(), @r###"
     NoSuchRevision {
@@ -1677,7 +1677,7 @@ fn test_evaluate_expression_git_head(use_git: bool) {
 
     // Can get git head when it's not set
     assert_eq!(resolve_commit_ids(mut_repo, "git_head()"), vec![]);
-    mut_repo.set_git_head(RefTarget::Normal(commit1.id().clone()));
+    mut_repo.set_git_head_target(Some(RefTarget::Normal(commit1.id().clone())));
     assert_eq!(
         resolve_commit_ids(mut_repo, "git_head()"),
         vec![commit1.id().clone()]

lib/tests/test_view.rs

@@ -442,19 +442,19 @@ fn test_merge_views_git_heads() {
     let mut tx0 = repo.start_transaction(&settings, "test");
     let tx0_head = write_random_commit(tx0.mut_repo(), &settings);
     tx0.mut_repo()
-        .set_git_head(RefTarget::Normal(tx0_head.id().clone()));
+        .set_git_head_target(Some(RefTarget::Normal(tx0_head.id().clone())));
     let repo = tx0.commit();
 
     let mut tx1 = repo.start_transaction(&settings, "test");
     let tx1_head = write_random_commit(tx1.mut_repo(), &settings);
     tx1.mut_repo()
-        .set_git_head(RefTarget::Normal(tx1_head.id().clone()));
+        .set_git_head_target(Some(RefTarget::Normal(tx1_head.id().clone())));
     tx1.commit();
 
     let mut tx2 = repo.start_transaction(&settings, "test");
     let tx2_head = write_random_commit(tx2.mut_repo(), &settings);
     tx2.mut_repo()
-        .set_git_head(RefTarget::Normal(tx2_head.id().clone()));
+        .set_git_head_target(Some(RefTarget::Normal(tx2_head.id().clone())));
     tx2.commit();
 
     let repo = repo.reload_at_head(&settings).unwrap();

src/cli_util.rs

@@ -796,7 +796,7 @@ impl WorkspaceCommandHelper {
                 let new_git_commit_id = Oid::from_bytes(first_parent_id.as_bytes()).unwrap();
                 let new_git_commit = git_repo.find_commit(new_git_commit_id)?;
                 git_repo.reset(new_git_commit.as_object(), git2::ResetType::Mixed, None)?;
-                mut_repo.set_git_head(RefTarget::Normal(first_parent_id));
+                mut_repo.set_git_head_target(Some(RefTarget::Normal(first_parent_id)));
             }
         } else {
             // The workspace was removed (maybe the user undid the