Merge: Pass Branch with pygit2 1.17.1+

This will produce a better MERGE_MSG.
See libgit2/pygit2#1348

Author: jorio

gitfourchette/tasks/branchtasks.py

@@ -583,7 +583,10 @@ def flow(self, them: str):
             self.effects |= TaskEffects.Workdir
             self.jumpTo = NavLocator.inWorkdir()
 
-            self.repo.merge(target)
+            if pygit2_version_at_least("1.17.1", raise_error=False, feature_name="git_annotated_commit_from_ref"):
+                self.repo.merge(theirBranch)
+            else:  # pragma: no cover
+                self.repo.merge(target)
             self.repoModel.prefs.draftCommitMessage = self.repo.message_without_conflict_comments
 
             self.postStatus = _("Merging {0} into {1}.", tquo(theirShorthand), tquo(myShorthand))

test/test_tasks_branch.py

@@ -916,7 +916,10 @@ def testMergeCausesConflicts(tempDir, mainWindow):
     acceptQMessageBox(rw, "empty commit")
     commitDialog: CommitDialog = findQDialog(rw, "commit")
     preparedMessage = commitDialog.getFullMessage()
-    assert re.match(r"Merge commit.+1b2bae55", preparedMessage)
+    if pygit2_version_at_least("1.17.1", raise_error=False, feature_name="git_annotated_commit_from_ref"):
+        assert re.match(r"Merge branch.+branch-conflicts", commitDialog.getFullMessage())
+    else:
+        assert re.match(r"Merge commit.+1b2bae55", preparedMessage)
     assert "Conflicts" not in preparedMessage
     assert "#" not in preparedMessage