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

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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>

options/locale/locale_en-US.json

@@ -1796,6 +1796,7 @@
   "repo.pulls.remove_prefix": "Remove <strong>%s</strong> prefix",
   "repo.pulls.data_broken": "This pull request is broken due to missing fork information.",
   "repo.pulls.files_conflicted": "This pull request has changes conflicting with the target branch.",
+  "repo.pulls.files_conflicted_no_listed_files": "(No conflicting files listed)",
   "repo.pulls.is_checking": "Checking for merge conflicts…",
   "repo.pulls.is_ancestor": "This branch is already included in the target branch. There is nothing to merge.",
   "repo.pulls.is_empty": "The changes on this branch are already on the target branch. This will be an empty commit.",