Merge bitcoin-core/secp256k1#1199: ci: Minor improvements inspired by Bitcoin Core

c2415866c7a6769cb29e3db6c5312c1255b37083 ci: Don't fetch git history (Tim Ruffing)
0ecf3188515e46b4da5580b4b9805d2cb927eb91 ci: Use remote pull/merge ref instead of local git merge (Tim Ruffing)

Pull request description:

  This steals two recent CI improvements from bitcoin/bitcoin.  See individual commit messages.

ACKs for top commit:
  sipa:
    utACK c2415866c7a6769cb29e3db6c5312c1255b37083

Tree-SHA512: 966130f45767c6bee8bc041d7e90a3166591a54c7cfccdcf4dff99aa4f6ccc2d02544fa7dca9fd020241349775da3cbd9bdbb041fcdd32de7426efd9dcc9c7f8

.cirrus.yml

@@ -1,4 +1,6 @@
 env:
+  ### cirrus config
+  CIRRUS_CLONE_DEPTH: 1
   ### compiler options
   HOST:
   WRAPPER_CMD:
@@ -53,11 +55,11 @@ cat_logs_snippet: &CAT_LOGS
 
 merge_base_script_snippet: &MERGE_BASE
   merge_base_script:
-    - if [ "$CIRRUS_PR" = "" ]; then exit 0; fi
-    - git fetch $CIRRUS_REPO_CLONE_URL $CIRRUS_BASE_BRANCH
     - git config --global user.email "ci@ci.ci"
     - git config --global user.name "ci"
-    - git merge FETCH_HEAD  # Merge base to detect silent merge conflicts
+    - if [ "$CIRRUS_PR" = "" ]; then exit 0; fi
+    - git fetch --depth=1 $CIRRUS_REPO_CLONE_URL "pull/${CIRRUS_PR}/merge"
+    - git checkout FETCH_HEAD  # Use merged changes to detect silent merge conflicts
 
 linux_container_snippet: &LINUX_CONTAINER
   container: