diff --git a/.cirrus.yml b/.cirrus.yml
index b37fce70020..f5874744b52 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -27,6 +27,13 @@ env:  # Global defaults
 # The above machine types are matched to each task by their label. Refer to the
 # Cirrus CI docs for more details.
 #
+# When a contributor maintains a fork of the repo, any pull request they make
+# to their own fork, or to the main repository, will trigger two CI runs:
+# one for the branch push and one for the pull request.
+# This can be avoided by setting SKIP_BRANCH_PUSH=true as a custom env variable
+# in Cirrus repository settings, accessible from
+# https://cirrus-ci.com/github/my-organization/my-repository
+#
 # On machines that are persisted between CI jobs, RESTART_CI_DOCKER_BEFORE_RUN=1
 # ensures that previous containers and artifacts are cleared before each run.
 # This requires installing Podman instead of Docker.
@@ -59,7 +66,10 @@ env:  # Global defaults
 
 # https://cirrus-ci.org/guide/tips-and-tricks/#sharing-configuration-between-tasks
 filter_template: &FILTER_TEMPLATE
-  skip: $CIRRUS_REPO_FULL_NAME == "bitcoin-core/gui" && $CIRRUS_PR == ""  # No need to run on the read-only mirror, unless it is a PR. https://cirrus-ci.org/guide/writing-tasks/#conditional-task-execution
+  # Allow forks to specify SKIP_BRANCH_PUSH=true and skip CI runs when a branch is pushed,
+  # but still run CI when a PR is created.
+  # https://cirrus-ci.org/guide/writing-tasks/#conditional-task-execution
+  skip: $SKIP_BRANCH_PUSH == "true" && $CIRRUS_PR == ""
   stateful: false  # https://cirrus-ci.org/guide/writing-tasks/#stateful-tasks
 
 base_template: &BASE_TEMPLATE
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 54795332e8a..ab9704c0af0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -58,7 +58,13 @@ jobs:
           # and the ^ prefix is used to exclude these parents and all their
           # ancestors from the rev-list output as described in:
           # https://git-scm.com/docs/git-rev-list
-          echo "TEST_BASE=$(git rev-list -n$((${{ env.MAX_COUNT }} + 1)) --reverse HEAD ^$(git rev-list -n1 --merges HEAD)^@ | head -1)" >> "$GITHUB_ENV"
+          MERGE_BASE=$(git rev-list -n1 --merges HEAD)
+          EXCLUDE_MERGE_BASE_ANCESTORS=
+          # MERGE_BASE can be empty due to limited fetch-depth
+          if test -n "$MERGE_BASE"; then
+            EXCLUDE_MERGE_BASE_ANCESTORS=^${MERGE_BASE}^@
+          fi
+          echo "TEST_BASE=$(git rev-list -n$((${{ env.MAX_COUNT }} + 1)) --reverse HEAD $EXCLUDE_MERGE_BASE_ANCESTORS | head -1)" >> "$GITHUB_ENV"
       - run: |
           sudo apt-get update
           sudo apt-get install clang ccache build-essential libtool autotools-dev automake pkg-config bsdmainutils python3-zmq libevent-dev libboost-dev libsqlite3-dev libdb++-dev systemtap-sdt-dev libminiupnpc-dev libnatpmp-dev qtbase5-dev qttools5-dev qttools5-dev-tools qtwayland5 libqrencode-dev -y