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>

services/pull/patch_unmerged.go

@@ -63,7 +63,7 @@ func readUnmergedLsFileLines(ctx context.Context, tmpBasePath string, outputChan
 	lsFilesReader, lsFilesReaderClose := cmd.MakeStdoutPipe()
 	defer lsFilesReaderClose()
 	err := cmd.WithDir(tmpBasePath).
-		WithPipelineFunc(func(_ gitcmd.Context) error {
+		WithPipelineFunc(func(gitcmd.Context) error {
 			bufferedReader := bufio.NewReader(lsFilesReader)
 
 			for {