fix: clear stale commit message after an external operation ends

When a rebase, merge, cherry-pick or revert is in progress, the working
copy auto-fills the commit message box from `MERGE_MSG`,
`rebase-merge/message` or the commit being replayed. If that operation
then finishes or is aborted outside SourceGit (e.g. from the terminal),
the loaded text stayed in the box, got persisted as the last commit
message on close, and reappeared on every later open of the repository.

Remember which message was auto-loaded and clear it once the in-progress
state disappears, unless the user has edited the text in the meantime.

Signed-off-by: Daniil Tatianin <99danilt@gmail.com>

Author: d-tatianin

src/ViewModels/WorkingCopy.cs

@@ -807,6 +807,12 @@ private void UpdateInProgressState()
             if (_inProgressContext != null && _inProgressContext.GetType() == oldType && !string.IsNullOrEmpty(_commitMessage))
                 return;
 
+            // The operation that filled the commit message ended outside SourceGit (for example, a
+            // rebase or merge finished/aborted from the terminal). Drop the stale message unless the
+            // user has edited it since it was loaded.
+            if (_inProgressContext == null && oldType != null && IsAutoLoadedCommitMessage())
+                CommitMessage = string.Empty;
+
             if (LoadCommitMessageFromFile(Path.Combine(_repo.GitDir, "MERGE_MSG")))
                 return;
 
@@ -816,7 +822,7 @@ private void UpdateInProgressState()
             if (LoadCommitMessageFromFile(Path.Combine(_repo.GitDir, "rebase-merge", "message")))
                 return;
 
-            CommitMessage = new Commands.QueryCommitFullMessage(_repo.FullPath, rebasing.StoppedAt.SHA).GetResult();
+            SetAutoLoadedCommitMessage(new Commands.QueryCommitFullMessage(_repo.FullPath, rebasing.StoppedAt.SHA).GetResult());
         }
 
         private bool LoadCommitMessageFromFile(string file)
@@ -828,10 +834,22 @@ private bool LoadCommitMessageFromFile(string file)
             if (string.IsNullOrEmpty(msg))
                 return false;
 
-            CommitMessage = msg;
+            SetAutoLoadedCommitMessage(msg);
             return true;
         }
 
+        private void SetAutoLoadedCommitMessage(string msg)
+        {
+            _autoLoadedCommitMessage = msg;
+            CommitMessage = msg;
+        }
+
+        private bool IsAutoLoadedCommitMessage()
+        {
+            return !string.IsNullOrEmpty(_autoLoadedCommitMessage) &&
+                string.Equals(_autoLoadedCommitMessage, _commitMessage, StringComparison.Ordinal);
+        }
+
         private void SetDetail(Models.Change change, bool isUnstaged)
         {
             if (_isLoadingData)
@@ -942,5 +960,6 @@ public ChangeInTree(Models.Change change)
 
         private bool _hasUnsolvedConflicts = false;
         private InProgressContext _inProgressContext = null;
+        private string _autoLoadedCommitMessage = null;
     }
 }