Specify default branch for git submodules.

[dbabokin]

Missing default branches causes fails if the project is imported through CMake's FetchContent mechanism. It checks out the default branch and then switches to the pinned SHA. If the default branch is not specified, it assumes "origin/master", which doesn't always exist.

Here's the fail that I observe:

Submodule path 'ext/c4core/cmake': checked out 'e87e11f1a10af8aab4e0ceaade0bc7807f207eb1'
Submodule path 'ext/c4core/src/c4/ext/debugbreak': checked out '328e4abca3384cbd0a69e70f263cc7b2794bff09'
fatal: Needed a single revision
Unable to find current origin/master revision in submodule path 'src/c4/ext/fast_float'

The issue is reproduced only with relatively old git, for example with the one that come with Ubuntu 20.04.

[biojppm]

Thanks!