performance, building cache

[larshp]

no need to read database if the parent was calculated in the previous step

    lv_parent = zcl_ags_obj_commit=>get_instance(
      iv_repo = mv_repo
      iv_sha1 = iv_commit )->get( )-parent.

[larshp]

also need to reconsider the overall approach, have an example with 100 million rows in the cache table, see #92