Use merge tree to detect conflicts when possible (#36400)

In Git 2.38, the `merge-tree` command introduced the `--write-tree`
option, which works directly on bare repositories. In Git 2.40, a new parameter `--merge-base` introduced so we require Git 2.40 to use the merge tree feature.

This option produces the merged tree object ID, allowing us to perform
diffs between commits without creating a temporary repository. By
avoiding the overhead of setting up and tearing down temporary repos,
this approach delivers a notable performance improvement.

It also fixes a possible situation that conflict files might be empty
but it's a conflict status according to
https://git-scm.com/docs/git-merge-tree#_mistakes_to_avoid

Replace #35542

---------

Signed-off-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/git/repo_branch_nogogit.go

@@ -101,7 +101,7 @@ func WalkShowRef(ctx context.Context, repoPath string, extraArgs gitcmd.TrustedC
 	stdoutReader, stdoutReaderClose := cmd.MakeStdoutPipe()
 	defer stdoutReaderClose()
 	cmd.WithDir(repoPath).
-		WithPipelineFunc(func(c gitcmd.Context) error {
+		WithPipelineFunc(func(gitcmd.Context) error {
 			bufReader := bufio.NewReader(stdoutReader)
 			for i < skip {
 				_, isPrefix, err := bufReader.ReadLine()
@@ -165,7 +165,7 @@ func WalkShowRef(ctx context.Context, repoPath string, extraArgs gitcmd.TrustedC
 			return nil
 		})
 	err = cmd.RunWithStderr(ctx)
-	if errPipeline := gitcmd.ErrorAsPipeline(err); errPipeline != nil {
+	if errPipeline, ok := gitcmd.UnwrapPipelineError(err); ok {
 		return i, errPipeline // keep the old behavior: return pipeline error directly
 	}
 	return i, err