Shallow checkout of submodules

Hi,

I've noticed that in some cases shallow checkout of submodules checks out the entire history (and takes longer to fetch and make much larger size repo).

I created two repos to show the issue. https://github.com/pps83/submodule-test-ok https://github.com/pps83/submodule-test-bad

Both are effectively identical: a readme file with a shallow submodule. submodule-test-ok links to https://github.com/pps83/brotli-master-min/tree/1e6f9b4b4c98f6f99ba9a860cbb982346631df80 submodule-test-bad links to https://github.com/pps83/brotli-master/tree/1e6f9b4b4c98f6f99ba9a860cbb982346631df80

Note, the hashes are identical (repos are different `brotli-master-min` vs `brotli-master`).

If you checkout submodule-test-bad ``` git clone https://github.com/pps83/submodule-test-bad.git cd submodule-test-bad time git submodule update --progress --init --force cd .. ``` entire `submodule-test-bad` is 34MB and takes a long time to fetch (if you are in some remote location with slow internet).

If you checkout submodule-test-ok ``` git clone https://github.com/pps83/submodule-test-ok.git cd submodule-test-ok time git submodule update --progress --init --force cd .. ``` entire `submodule-test-ok` is only 2.9MB and takes a second to fetch. At the end the only difference is in the `.git` folders, while contents of these two repos are identical.

Not sure if that's a git issue, or github. But the issue happens because of the default branch settings in the repo. In short, shallow checkout of a submodule from a non-default branch on github results in a full checkout.

Thanks, Pavel