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/repository/gitgraph/graph.go

@@ -81,7 +81,7 @@ func GetCommitGraph(r *git.Repository, page, maxAllowedColors int, hidePRRefs bo
 				line := scanner.Bytes()
 				if bytes.IndexByte(line, '*') >= 0 {
 					if err := parser.AddLineToGraph(graph, row, line); err != nil {
-						return ctx.CancelWithCause(err)
+						return ctx.CancelPipeline(err)
 					}
 					break
 				}
@@ -92,7 +92,7 @@ func GetCommitGraph(r *git.Repository, page, maxAllowedColors int, hidePRRefs bo
 				row++
 				line := scanner.Bytes()
 				if err := parser.AddLineToGraph(graph, row, line); err != nil {
-					return ctx.CancelWithCause(err)
+					return ctx.CancelPipeline(err)
 				}
 			}
 			return scanner.Err()