[CI] Merge PR against its target branch instead of main (#19712)

ci/jenkins/templates/utils/Prepare.groovy.j2

@@ -15,12 +15,15 @@ def init_git() {
   )
 
   // Determine merge commit to use for all stages
-  if (env.BRANCH_NAME == 'main') {
-    // Only set upstream_revision to HEAD and skip merging to avoid a race with another commit merged to main.
-    update_upstream_revision("HEAD")
+  if (env.CHANGE_TARGET) {
+    // This is a PR build, so merge with the latest of the PR's target branch
+    // (e.g. main or a release branch like v0.25.0).
+    merge_with_target()
   } else {
-    // This is PR branch so merge with latest main.
-    merge_with_main()
+    // This is a branch build (main or a release branch). Only set
+    // upstream_revision to HEAD and skip merging to avoid a race with another
+    // commit merged to the branch.
+    update_upstream_revision("HEAD")
   }
 
   sh(
@@ -44,15 +47,16 @@ def update_upstream_revision(git_ref) {
   }
 }
 
-def merge_with_main() {
+def merge_with_target() {
+  def target = env.CHANGE_TARGET
   sh (
-    script: 'git fetch origin main',
-    label: 'Fetch upstream',
+    script: "git fetch origin ${target}",
+    label: "Fetch target branch ${target}",
   )
   update_upstream_revision("FETCH_HEAD")
   sh (
     script: "git -c user.name=TVM-Jenkins -c user.email=jenkins@tvm.apache.org merge ${upstream_revision}",
-    label: 'Merge to origin/main'
+    label: "Merge to origin/${target}"
   )
 }