Fix false backport detection for non-default branch workflows (#568)

* Fix false backport detection for non-default branch workflows

In Gitflow-style repos (e.g. MLJBase.jl) where the default branch is
`develop` and releases happen on `master`, TagBot incorrectly marked
every release as a backport because `branches_of_commit` returned
`master` (a non-default branch).

The backport flag now relies solely on `_is_backport()`, which checks
whether a higher-version tag already exists—the actual semantic
definition of a backport.

Fixes #566

Co-authored-by: Claude <noreply@anthropic.com>

* fix flake8

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: IanButterworth

tagbot/action/changelog.py

@@ -422,7 +422,7 @@ def _collect_data(self, version_tag: str, sha: str) -> Dict[str, object]:
         return {
             "compare_url": compare,
             "custom": self._custom_release_notes(version_tag),
-            "backport": self._is_backport(version_tag) or bool(release_branches),
+            "backport": self._is_backport(version_tag),
             "issues": [self._format_issue(i) for i in issues],
             "package": self._repo._project("name"),
             "previous_release": prev_tag,

test/action/test_changelog.py

@@ -415,6 +415,36 @@ def test_collect_data_backport():
     )
 
 
+def test_collect_data_non_default_branch_not_backport():
+    """Commit on non-default branch (Gitflow) with no higher tag."""
+    c = _changelog()
+    c._repo._repo = Mock(full_name="A/B.jl", html_url="https://github.com/A/B.jl")
+    c._repo._project = Mock(return_value="B")
+    c._repo.is_version_yanked = Mock(return_value=False)
+    c._repo.branches_of_commit = Mock(return_value=["master"])
+    semver_prev = Mock(
+        tag_name="v0.18.6", created_at=datetime(2023, 1, 1, tzinfo=timezone.utc)
+    )
+    chrono_prev = Mock(
+        tag_name="v0.18.6", created_at=datetime(2023, 1, 1, tzinfo=timezone.utc)
+    )
+    c._previous_release = Mock(return_value=semver_prev)
+    c._previous_release_chronological = Mock(return_value=chrono_prev)
+    c._is_backport = Mock(return_value=False)
+    commit_date = datetime(2023, 4, 1, tzinfo=timezone.utc)
+    commit = Mock(commit=Mock(author=Mock(date=commit_date)))
+    c._repo._repo.get_commit = Mock(return_value=commit)
+    c._issues = Mock(return_value=[])
+    c._pulls_on_branches = Mock(return_value=[])
+    c._custom_release_notes = Mock(return_value=None)
+
+    data = c._collect_data("v0.18.7", "abcdef")
+
+    assert data["backport"] is False
+    assert data["previous_release"] == "v0.18.6"
+    assert data["compare_url"] == "https://github.com/A/B.jl/compare/v0.18.6...v0.18.7"
+
+
 def test_pulls_on_branches():
     c = _changelog()
     pr_main = Mock(spec=PullRequest)