ci: Use remote pull/merge ref instead of local git merge

The merge strategy on the remote may be different than the local one.
This may cause local merges to be different or fail completely. Fix this
by using the result of the remote merge.

Github-Pull: bitcoin/bitcoin#26202
Rebased-From: fad7281d78

.cirrus.yml

@@ -30,8 +30,8 @@ base_template: &BASE_TEMPLATE
     - git config --global user.email "ci@ci.ci"
     - git config --global user.name "ci"
     - if [ "$CIRRUS_PR" = "" ]; then exit 0; fi
-    - git fetch $CIRRUS_REPO_CLONE_URL $CIRRUS_BASE_BRANCH
-    - git merge FETCH_HEAD  # Merge base to detect silent merge conflicts
+    - git fetch $CIRRUS_REPO_CLONE_URL "pull/${CIRRUS_PR}/merge"
+    - git checkout FETCH_HEAD  # Use merged changes to detect silent merge conflicts
 
 main_template: &MAIN_TEMPLATE
   timeout_in: 120m  # https://cirrus-ci.org/faq/#instance-timed-out