ci: correctly detect outdated branches

tools/ci-changed-builds.sh

@@ -1,7 +1,12 @@
 #!/bin/bash
 set -euo pipefail
 IFS=$'\n'
-filenames="$(git diff-tree -r --no-commit-id --name-status "$(git merge-base "$1" HEAD)" HEAD | ( grep -P '^[AM]\t(xep-[0-9]{4}|inbox/[^/]+)\.xml$' || true) | cut -f2)"
+if ! merge_base="$(git merge-base "$1" HEAD)"; then
+    echo 'Failed to find merge base to detect changed files' >&2
+    echo 'This indicates that your branch is too old and needs to be rebased' >&2
+    exit 2
+fi
+filenames="$(git diff-tree -r --no-commit-id --name-status "$merge_base" HEAD | ( grep -P '^[AM]\t(xep-[0-9]{4}|inbox/[^/]+)\.xml$' || true) | cut -f2)"
 if [ -z "$filenames" ]; then
     exit 0
 fi