Update jj edit <commit> to add commit into view heads if not already

`jj new <commit>` automatically adds the checked out commits into the view head ids. However,
`jj edit` does not.

To reproduce:
```
jj git init test
cd test
jj commit -m "my commit"
jj log -r @- -T commit_id # Save the id
jj abandon -r @-
jj edit <saved_id>

jj log -r :: # Does not show the currently editing commit 
```

CHANGELOG.md

@@ -41,6 +41,8 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
  * Fixed panic when parsing invalid conflict markers of a particular form.
    ([#2611](https://github.com/martinvonz/jj/pull/2611))
 
+ * Editing a hidden commit now makes it visible.
+
 ## [0.21.0] - 2024-09-04
 
 ### Breaking changes

lib/src/repo.rs

@@ -1375,6 +1375,7 @@ impl MutableRepo {
         commit: &Commit,
     ) -> Result<(), EditCommitError> {
         self.maybe_abandon_wc_commit(&workspace_id)?;
+        self.add_head(commit)?;
         self.set_wc_commit(workspace_id, commit.id().clone())
             .map_err(|RewriteRootCommit| EditCommitError::RewriteRootCommit)
     }

lib/tests/test_mut_repo.rs

@@ -318,6 +318,28 @@ fn test_edit_initial() {
     );
 }
 
+#[test]
+fn test_edit_hidden_commit() {
+    // Test that MutableRepo::edit() edits a hidden commit and updates
+    // the view head ids.
+    let settings = testutils::user_settings();
+    let test_repo = TestRepo::init();
+    let repo = &test_repo.repo;
+
+    let mut tx = repo.start_transaction(&settings);
+    let wc_commit = write_random_commit(tx.repo_mut(), &settings);
+
+    // Intentionally not doing tx.commit, so the commit id is not tracked
+    // in the view head ids.
+
+    let mut tx = repo.start_transaction(&settings);
+    let ws_id = WorkspaceId::default();
+    tx.repo_mut().edit(ws_id.clone(), &wc_commit).unwrap();
+    let repo = tx.commit("test");
+    assert_eq!(repo.view().get_wc_commit_id(&ws_id), Some(wc_commit.id()));
+    assert_eq!(*repo.view().heads(), hashset! {wc_commit.id().clone()});
+}
+
 #[test]
 fn test_add_head_success() {
     // Test that MutableRepo::add_head() adds the head, and that it's still there