Merge bitcoin/bitcoin#26202: ci: Use remote pull/merge ref instead of local git merge

fad7281 ci: Use remote pull/merge ref instead of local git merge (MacroFake)

Pull request description:

  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.

  Fixes bitcoin/bitcoin#26163

ACKs for top commit:
  hebasto:
    ACK fad7281, I regularly use the same commands locally.

Tree-SHA512: 0febbf5db8c1536e31b374a7599a92037ca814174809075f42c7c7c4e1daaab5b3df09cf82f2de0d1e847c41eb30e770daaf7a85287f5d8d43ebd642d1234d3c

Author: MacroFake

.cirrus.yml

@@ -30,8 +30,8 @@ base_template: &BASE_TEMPLATE
     - git config --global user.email "[email protected]"
     - 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