diff --git a/.gitlab-ci/run-check-todos.sh b/.gitlab-ci/run-check-todos.sh
index 8af102739..8a6a9bfda 100755
--- a/.gitlab-ci/run-check-todos.sh
+++ b/.gitlab-ci/run-check-todos.sh
@@ -2,11 +2,17 @@
 
 set -e
 
+ancestor_horizon=28  # days (4 weeks)
+
 # We need to add a new remote for the upstream target branch, since this script
 # could be running in a personal fork of the repository which has out of date
 # branches.
+#
+# Limit the fetch to a certain date horizon to limit the amount of data we get.
+# If the branch was forked from origin/master before this horizon, it should
+# probably be rebased.
 git remote add upstream https://gitlab.gnome.org/GNOME/glib.git
-git fetch upstream
+git fetch --shallow-since="$(date --date="${ancestor_horizon} days ago" +%Y-%m-%d)" upstream
 
 # Work out the newest common ancestor between the detached HEAD that this CI job
 # has checked out, and the upstream target branch (which will typically be
@@ -14,4 +20,11 @@ git fetch upstream
 # `${CI_MERGE_REQUEST_TARGET_BRANCH_NAME}` is only defined if we’re running in
 # a merge request pipeline; fall back to `${CI_DEFAULT_BRANCH}` otherwise.
 newest_common_ancestor_sha=$(diff --old-line-format='' --new-line-format='' <(git rev-list --first-parent "upstream/${CI_MERGE_REQUEST_TARGET_BRANCH_NAME:-${CI_DEFAULT_BRANCH}}") <(git rev-list --first-parent HEAD) | head -1)
+if [ -z "${newest_common_ancestor_sha}" ]; then
+    echo "Couldn’t find common ancestor with upstream master. This typically"
+    echo "happens if you branched from master a long time ago. Please update"
+    echo "your clone, rebase, and re-push your branch."
+    exit 1
+fi
+
 ./.gitlab-ci/check-todos.py "${newest_common_ancestor_sha}"
diff --git a/.gitlab-ci/run-style-check-diff.sh b/.gitlab-ci/run-style-check-diff.sh
index 1f7b8c125..b308e7679 100755
--- a/.gitlab-ci/run-style-check-diff.sh
+++ b/.gitlab-ci/run-style-check-diff.sh
@@ -2,21 +2,35 @@
 
 set -e
 
+ancestor_horizon=28  # days (4 weeks)
+
 # Wrap everything in a subshell so we can propagate the exit status.
 (
 
 # We need to add a new remote for the upstream target branch, since this script
 # could be running in a personal fork of the repository which has out of date
 # branches.
+#
+# Limit the fetch to a certain date horizon to limit the amount of data we get.
+# If the branch was forked from origin/master before this horizon, it should
+# probably be rebased.
 git remote add upstream https://gitlab.gnome.org/GNOME/glib.git
-git fetch upstream
+git fetch --shallow-since="$(date --date="${ancestor_horizon} days ago" +%Y-%m-%d)" upstream
 
 # Work out the newest common ancestor between the detached HEAD that this CI job
 # has checked out, and the upstream target branch (which will typically be
 # `upstream/master` or `upstream/glib-2-62`).
+#
 # `${CI_MERGE_REQUEST_TARGET_BRANCH_NAME}` is only defined if we’re running in
 # a merge request pipeline; fall back to `${CI_DEFAULT_BRANCH}` otherwise.
 newest_common_ancestor_sha=$(diff --old-line-format='' --new-line-format='' <(git rev-list --first-parent "upstream/${CI_MERGE_REQUEST_TARGET_BRANCH_NAME:-${CI_DEFAULT_BRANCH}}") <(git rev-list --first-parent HEAD) | head -1)
+if [ -z "${newest_common_ancestor_sha}" ]; then
+    echo "Couldn’t find common ancestor with upstream master. This typically"
+    echo "happens if you branched from master a long time ago. Please update"
+    echo "your clone, rebase, and re-push your branch."
+    exit 1
+fi
+
 git diff -U0 --no-color "${newest_common_ancestor_sha}" | ./clang-format-diff.py -binary "clang-format-7" -p1
 
 )