Store the commit message in the correct file

and
Replace commit message in an undo-able way
Disable change of message and amend unless Normal

GitCommands/CommitMessageManager.cs

@@ -0,0 +1,124 @@
+using System.IO.Abstractions;
+using System.Text;
+
+namespace GitCommands
+{
+    public interface ICommitMessageManager
+    {
+        /// <summary>
+        /// Reads/stores whether the previous commit shall be amended (if AppSettings.RememberAmendCommitState).
+        /// </summary>
+        bool AmendState { get; set; }
+
+        /// <summary>
+        /// The pathname of the file where a prepared (non-merge) commit message is stored.
+        /// </summary>
+        string CommitMessagePath { get; }
+
+        /// <summary>
+        /// Returns whether .git/MERGE_MSG exists.
+        /// </summary>
+        bool IsMergeCommit { get; }
+
+        /// <summary>
+        /// Reads/stores the prepared commit message from/in .git/MERGE_MSG if it exists or else in .git/COMMITMESSAGE.
+        /// </summary>
+        string MergeOrCommitMessage { get; set; }
+
+        /// <summary>
+        /// Deletes .git/COMMITMESSAGE and the file with the AmendState.
+        /// </summary>
+        void ResetCommitMessage();
+    }
+
+    public sealed class CommitMessageManager : ICommitMessageManager
+    {
+        private readonly string _amendSaveStatePath;
+        private readonly string _commitMessagePath;
+        private readonly string _mergeMessagePath;
+
+        private Encoding _commitEncoding;
+        private IFileSystem _fileSystem;
+
+        public CommitMessageManager(string workingDirGitDir, Encoding commitEncoding)
+            : this(workingDirGitDir, commitEncoding, new FileSystem())
+        {
+        }
+
+        private CommitMessageManager(string workingDirGitDir, Encoding commitEncoding, IFileSystem fileSystem)
+        {
+            _fileSystem = fileSystem;
+            _commitEncoding = commitEncoding;
+            _amendSaveStatePath = GetFilePath(workingDirGitDir, "GitExtensions.amend");
+            _commitMessagePath = GetFilePath(workingDirGitDir, "COMMITMESSAGE");
+            _mergeMessagePath = GetFilePath(workingDirGitDir, "MERGE_MSG");
+        }
+
+        public bool AmendState
+        {
+            get
+            {
+                bool amendState = false;
+
+                if (AppSettings.RememberAmendCommitState && _fileSystem.File.Exists(_amendSaveStatePath))
+                {
+                    bool.TryParse(_fileSystem.File.ReadAllText(_amendSaveStatePath), out amendState);
+                }
+
+                return amendState;
+            }
+            set
+            {
+                if (AppSettings.RememberAmendCommitState && value)
+                {
+                    _fileSystem.File.WriteAllText(_amendSaveStatePath, true.ToString());
+                }
+                else
+                {
+                    _fileSystem.File.Delete(_amendSaveStatePath);
+                }
+            }
+        }
+
+        public string CommitMessagePath => _commitMessagePath;
+
+        public bool IsMergeCommit => _fileSystem.File.Exists(_mergeMessagePath);
+
+        public string MergeOrCommitMessage
+        {
+            get
+            {
+                var (file, exists) = GetMergeOrCommitMessagePath();
+                return exists ? _fileSystem.File.ReadAllText(file, _commitEncoding) : string.Empty;
+            }
+            set
+            {
+                _fileSystem.File.WriteAllText(GetMergeOrCommitMessagePath().FilePath, value ?? string.Empty, _commitEncoding);
+            }
+        }
+
+        public void ResetCommitMessage()
+        {
+            _fileSystem.File.Delete(_commitMessagePath);
+            _fileSystem.File.Delete(_amendSaveStatePath);
+        }
+
+        private string GetFilePath(string workingDirGitDir, string fileName) => _fileSystem.Path.Combine(workingDirGitDir, fileName);
+
+        private (string FilePath, bool FileExists) GetMergeOrCommitMessagePath()
+        {
+            if (IsMergeCommit)
+            {
+                return (_mergeMessagePath, FileExists: true);
+            }
+
+            return (_commitMessagePath, _fileSystem.File.Exists(_commitMessagePath));
+        }
+
+        internal class TestAccessor
+        {
+            internal static CommitMessageManager Construct(string workingDirGitDir, Encoding commitEncoding, IFileSystem fileSystem)
+                => new CommitMessageManager(workingDirGitDir, commitEncoding, fileSystem);
+        }
+    }
+}

GitCommands/Git/GitModule.cs

@@ -868,16 +868,6 @@ public string GetCommitCountString(string from, string to)
                 (added > 0 ? ("+" + added) : "");
         }
 
-        public string GetMergeMessage()
-        {
-            var file = GetGitDirectory() + "MERGE_MSG";
-
-            return
-                File.Exists(file)
-                    ? File.ReadAllText(file)
-                    : "";
-        }
-
         public void RunGitK()
         {
             if (EnvUtils.RunningOnUnix())

GitCommands/GitCommands.csproj

@@ -182,7 +182,7 @@
     <Compile Include="Git\GitSubmoduleStatus.cs" />
     <Compile Include="LockableNotifier.cs" />
     <Compile Include="Logging\CommandLog.cs" />
-    <Compile Include="CommitHelper.cs" />
+    <Compile Include="CommitMessageManager.cs" />
     <Compile Include="Config\ConfigFile.cs" />
     <Compile Include="Config\ConfigSection.cs" />
     <Compile Include="Git\CommandCache.cs" />