refactor: testing merge

- Check exitcode of `git merge-tree --write-tree <branch1> <branch2>` command instead of parsing the output of `git merge-tree <base> <src> <dest>` to testing merge
- Requires git >= 2.38.0

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

Author: love-linger

src/Commands/MergeBase.cs

@@ -1,29 +1,32 @@
-using System;
+using System.Diagnostics;
 using System.Threading.Tasks;
 
 namespace SourceGit.Commands
 {
     public class MergeTree : Command
     {
-        public MergeTree(string repo, string baseTree, string source, string dest)
+        public MergeTree(string repo, string source, string dest)
         {
             WorkingDirectory = repo;
-            Context = repo;
-            RaiseError = false;
-            Args = $"merge-tree {baseTree} {source} {dest}";
+            Args = $"merge-tree --write-tree {source} {dest}";
         }
 
         public async Task<bool> CheckAsync()
         {
-            var rs = await ReadToEndAsync().ConfigureAwait(false);
-            if (!rs.IsSuccess)
-                return false;
+            using var proc = new Process();
+            proc.StartInfo = CreateGitStartInfo(false);
 
-            var stdout = rs.StdOut;
-            return !stdout.Contains("\n+>>>>>>>", StringComparison.Ordinal) &&
-                !stdout.Contains("\n+<<<<<<<", StringComparison.Ordinal) &&
-                !stdout.Contains("\n->>>>>>>", StringComparison.Ordinal) &&
-                !stdout.Contains("\n-<<<<<<<", StringComparison.Ordinal);
+            try
+            {
+                proc.Start();
+                await proc.WaitForExitAsync().ConfigureAwait(false);
+                return proc.ExitCode == 0;
+            }
+            catch
+            {
+                // Ignore any exceptions and just return false
+                return false;
+            }
         }
     }
 }

src/Commands/MergeTree.cs

@@ -16,5 +16,10 @@ public static class GitVersions
         ///     The minimal version of Git that supports the `stash push` command with the `--staged` option.
         /// </summary>
         public static readonly System.Version STASH_PUSH_ONLY_STAGED = new(2, 35, 0);
+
+        /// <summary>
+        ///     The minimal version of Git that supports the `git merge-tree --write-tree` command, which is used for testing merge results without actually performing a merge.
+        /// </summary>
+        public static readonly System.Version TESTING_MERGE = new(2, 38, 0);
     }
 }

src/Models/GitVersions.cs

@@ -149,26 +149,18 @@ private Models.MergeMode AutoSelectMergeMode()
 
         private void Test()
         {
+            if (Native.OS.GitVersion < Models.GitVersions.TESTING_MERGE)
+                return;
+
             TestingState = MergeTestingState.Testing;
 
             Task.Run(async () =>
             {
-                var mergeBase = await new Commands.MergeBase(_repo.FullPath, _sourceName, Into)
-                    .GetResultAsync()
+                var ok = await new Commands.MergeTree(_repo.FullPath, _sourceName, Into)
+                    .CheckAsync()
                     .ConfigureAwait(false);
 
-                if (!string.IsNullOrEmpty(mergeBase))
-                {
-                    var ok = await new Commands.MergeTree(_repo.FullPath, mergeBase, _sourceName, Into)
-                        .CheckAsync()
-                        .ConfigureAwait(false);
-
-                    Dispatcher.UIThread.Post(() => TestingState = ok ? MergeTestingState.NoConflicts : MergeTestingState.WillCauseConflicts);
-                }
-                else
-                {
-                    Dispatcher.UIThread.Post(() => TestingState = MergeTestingState.Disabled);
-                }
+                Dispatcher.UIThread.Post(() => TestingState = ok ? MergeTestingState.NoConflicts : MergeTestingState.WillCauseConflicts);
             });
         }