gc/recursive-fetch-with-unused-submodules (was Re: What's cooking in git.git (Mar 2022, #03; Mon, 14))

> * gc/recursive-fetch-with-unused-submodules (2022-03-07) 10 commits
>  - submodule: fix latent check_has_commit() bug
>  - fetch: fetch unpopulated, changed submodules
>  - submodule: move logic into fetch_task_create()
>  - submodule: extract get_fetch_task()
>  - submodule: store new submodule commits oid_array in a struct
>  - submodule: inline submodule_commits() into caller
>  - submodule: make static functions read submodules from commits
>  - t5526: create superproject commits with test helper
>  - t5526: stop asserting on stderr literally
>  - t5526: introduce test helper to assert on fetches
>
>  When "git fetch --recurse-submodules" grabbed submodule commits
>  that would be needed to recursively check out newly fetched commits
>  in the superproject, it only paid attention to submodules that are
>  in the current checkout of the superproject.  We now do so for all
>  submodules that have been run "git submodule init" on.
>
>  Expecting a reroll.
>  cf. <kl6ly21p2q00.fsf@chooglen-macbookpro.roam.corp.google.com>
>  source: <20220308001433.94995-1-chooglen@google.com>