enhance: always try to load commit message from $GIT_DIR/MERGE_MSG (sourcegit-scm#1915)

Signed-off-by: leo <longshuang@msn.cn>

Author: love-linger

src/ViewModels/WorkingCopy.cs

@@ -776,7 +776,10 @@ private void UpdateInProgressState()
                 InProgressContext = null;
 
             if (_inProgressContext == null)
+            {
+                LoadCommitMessageFromFile(Path.Combine(_repo.GitDir, "MERGE_MSG"));
                 return;
+            }
 
             if (_inProgressContext.GetType() == oldType && !string.IsNullOrEmpty(_commitMessage))
                 return;