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/git.go

@@ -32,6 +32,7 @@ type Features struct {
 	SupportedObjectFormats     []ObjectFormat // sha1, sha256
 	SupportCheckAttrOnBare     bool           // >= 2.40
 	SupportCatFileBatchCommand bool           // >= 2.36, support `git cat-file --batch-command`
+	SupportGitMergeTree        bool           // >= 2.40 // we also need "--merge-base"
 }
 
 var defaultFeatures *Features
@@ -77,6 +78,7 @@ func loadGitVersionFeatures() (*Features, error) {
 	}
 	features.SupportCheckAttrOnBare = features.CheckVersionAtLeast("2.40")
 	features.SupportCatFileBatchCommand = features.CheckVersionAtLeast("2.36")
+	features.SupportGitMergeTree = features.CheckVersionAtLeast("2.40") // we also need "--merge-base"
 	return features, nil
 }