Failed shallow submodule clone for fixed SHA1 falls back to checking out HEAD

Hi,

this is using "git version 2.12.2.windows.2" on Windows / "git version 2.12.2-639-g584f897" on Linux.

I have configured my superproject with .gitmodules saying

---8<---
[submodule "src/funTest/resources/projects/external/jgnash"]
       path = src/funTest/resources/projects/external/jgnash
       url = https://github.com/ccavanaugh/jgnash.git
       shallow = true
---8<---

and configured the submodule to checkout commit 2aa4cce7d7fd46164030f2b4d244c4b89e77f722 [1]. When doing a fresh clone of my superproject via "git clone --recursive" I get

---8<--- error: Server does not allow request for unadvertised object 2aa4cce7d7fd46164030f2b4d244c4b89e77f722 Fetched in submodule path 'src/funTest/resources/projects/external/jgnash', but it did not contain 2aa4cce7d7fd464030f2b4d244c4b89e77f722. Direct fetching of that commit failed. ---8<---

So far so good, it simply seems that GitHub does not support allowReachableSHA1InWant [2]. The interesting bit is that my submodule checkout still ends up being shallow, but poiting to HEAD:

---8<---
$ cd src/funTest/resources/projects/external/jgnash
$ git log
commit 12036fffb6c620515edd96416363fd1749b5d003 (grafted, HEAD -> master, origin/master, origin/HEAD)
Author: Craig Cavanaugh <jgnash.devel@gmail.com>
Date:   Tue Apr 18 05:33:06 2017 -0400

    Fix typos
---8<---

Wouldn't it make more sense to unshallow the submodule clone in this case and checkout the configured commit 2aa4cce7d7fd46164030f2b4d244c4b89e77f722 afterwards? At least I'd be getting what I asked for in that case, even if at the cost of additional network traffic. Because if I read [3] correctly, the command line option belonging to "submodule.<name>.shallow" is called "--[no-]recommend-shallow", i.e. it's only a recommendation, to falling back to a full clone should be fine.

[1] https://github.com/ccavanaugh/jgnash/commit/2aa4cce7d7fd46164030f2b4d244c4b89e77f722 [2] https://git-scm.com/docs/git-config#git-config-uploadpackallowReachableSHA1InWant [3] https://git-scm.com/docs/git-submodule

Regards, Sebastian