Return created commits on merge and revert in transaction

This seems like the more natural thing to do. Adjusts the tests to
expect the new behavior.

Also, since the merge logic only optionally creates a new commit on the
destination branch, we can unconditionally return the destination HEAD in
`LakeFSTransaction.merge()`.

src/lakefs_spec/transaction.py

@@ -149,7 +149,7 @@ def commit(self, message: str, metadata: dict[str, str] | None = None) -> Refere
 
         return self.branch.commit(message, metadata=metadata)
 
-    def merge(self, source_ref: str | Branch, into: str | Branch) -> str:
+    def merge(self, source_ref: str | Branch, into: str | Branch) -> Commit:
         """
         Merge a branch into another branch in a repository.
 
@@ -166,17 +166,17 @@ def merge(self, source_ref: str | Branch, into: str | Branch) -> str:
 
         Returns
         -------
-        str
-            The ID of either the created merge commit, or the tip of the target branch.
+        Commit
+            Either the created merge commit, or the head commit of the target branch.
         """
         source = ensurebranch(source_ref, self.repository, self.fs.client)
         dest = ensurebranch(into, self.repository, self.fs.client)
 
         if any(dest.diff(source)):
-            return source.merge_into(dest)
-        return dest.head.get_commit().id
+            source.merge_into(dest)
+        return dest.head.get_commit()
 
-    def revert(self, branch: str | Branch, ref: ReferenceType, parent_number: int = 1) -> None:
+    def revert(self, branch: str | Branch, ref: ReferenceType, parent_number: int = 1) -> Commit:
         """
         Revert a previous commit on a branch.
 
@@ -190,13 +190,18 @@ def revert(self, branch: str | Branch, ref: ReferenceType, parent_number: int =
             If there are multiple parents to a commit, specify to which parent
             the commit should be reverted. ``parent_number = 1`` (the default)
             refers to the first parent commit of the current ``branch`` tip.
+
+        Returns
+        -------
+        Commit
+            The created revert commit.
         """
 
         b = ensurebranch(branch, self.repository, self.fs.client)
 
         ref_id = ref if isinstance(ref, str) else ref.id
         b.revert(ref_id, parent_number=parent_number)
-        return None
+        return b.head.get_commit()
 
     def rev_parse(self, ref: ReferenceType) -> Commit:
         """

tests/test_transactions.py

@@ -85,18 +85,16 @@ def test_transaction_revert(
     random_file = random_file_factory.make()
 
     lpath = str(random_file)
-    rpath = f"{repository.id}/{temp_branch.id}/{random_file.name}"
-
     message = f"Add file {random_file.name}"
 
-    with fs.transaction(repository, temp_branch) as tx:
+    with fs.transaction(repository, temp_branch, automerge=True) as tx:
         fs.put_file(lpath, f"{repository.id}/{tx.branch.id}/{random_file.name}")
         tx.commit(message=message)
-        tx.revert(temp_branch, temp_branch.head)
+        revert_commit = tx.revert(temp_branch, temp_branch.head)
 
-    head, head_tilde = list(temp_branch.log(max_amount=2))
-    assert head.message.startswith("Merge")
-    assert head_tilde.message.startswith("Revert")
+    # first commit should be the merge commit
+    assert temp_branch.head.get_commit().message.startswith("Merge")
+    assert revert_commit.message.startswith("Revert")
 
 
 def test_transaction_failure(
@@ -109,7 +107,6 @@ def test_transaction_failure(
 
     lpath = str(random_file)
     rpath = f"{repository.id}/{temp_branch.id}/{random_file.name}"
-
     message = f"Add file {random_file.name}"
 
     try: