codeintel: Enhance "Select Git object type" in policy configuration UI

Created by: efritz

Currently we allow types of GIT_TREE (branch) and GIT_TAG for global policies and those two plus GIT_COMMIT for repository policies.

Instead, we should allow GIT_COMMIT for both, but do not accept a pattern for it (use this as HEAD always). We currently don't need a way to retain code intel for a specific commit (I can think of reasons, but they're super edge-cased), but we do have a necessity to retain and index the tip of the default branch, which we can't currently express.

This change should be reflected in both the UI and graphql layer.

Current screenshots:

More context in comments in this issue.