In order to get tags, including in submodules, we use our own fetching
procedure on top of checkout@v2.
A problem occuring in about 1% of jobs was that some submodules inexplicably
did not have an "origin" remote configured. "git submodule sync"
configures the "origin" remote in those cases. No cause for the problem
was determined.
Besides keeping up to date on actions/checkout, @v2 is supposed to fix a bug
where "re-run" of a pull request would fail checking out the code.
