ci: increase fetch depth for changed file detection

This is required to accomodate branches which are slightly behind
master.

.gitlab-ci.yml

@@ -78,7 +78,7 @@ stages:
   script:
   - python3 tools/ci-restore-timestamps.py
   - make html inbox-html
-  - git fetch --depth=1 origin main
+  - git fetch --depth=50 origin main
   - bash tools/ci-changed-builds.sh origin/main
   rules:
   - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'