CI: gitlab: increase git clone depth to 100 (MR 5718)

Technically this option can be configured in the Gitlab UI, but I spent
about 3 hours debugging a CI failure caused by it being too low and
I don't want to risk forgetting this can happen later if/when someone
decides to lower it in the UI. By setting it here, we can have some
context/history about why.

This fixes the CI in !5688

Also see:
https://docs.gitlab.com/17.4/ee/user/project/repository/monorepos/index.html#shallow-cloning
[ci:skip-build]: already built successfully in CI

.gitlab-ci.yml

@@ -10,6 +10,11 @@ stages:
   - lint
   - build
   - autoupdate
+variables:
+  # Default of 20 is pretty small, sometimes MRs have >20 commits in them. 100
+  # seems like a reasonable balance. If this is too low, git merge-base (from ci/
+  # common.py) can fail
+  GIT_DEPTH: 100
   
 # This defines the rules for when a pipeline should run.
 workflow: