Fetch pinned RDM history deep enough for ancestry

Why: depth 1 makes prev_sha and sha unrelated shallow roots, so update reports a fork instead of an upgrade.
Evidence: SELF the pin-fetch credential test; CI run 2829 showed the fork message.
Impact: test-pg fetches 200 commits from each pin, not the full history.
Refs: .task/feature-svc-d3-real-scenarios.md
Co-authored-by: Cursor <cursoragent@cursor.com>

.gitea/workflows/ci.yaml

@@ -421,7 +421,9 @@ jobs:
           PY
           )
           [ "${#pins[@]}" -eq 3 ]
-          git -C rdm fetch --depth=1 origin "${pins[@]}"
+          # depth=1 会把三个 pin 变成互不相关的浅根，update 会把 prev→sha 误判成分叉。
+          # 当前 first-parent 距离是 39，200 留余量，仍不拉全史。
+          git -C rdm fetch --depth=200 origin "${pins[@]}"
           for rev in "${pins[@]}"; do
             git -C rdm checkout --detach "$rev"
             git -C rdm submodule update --init --recursive