This works by checking the $CIRCLE_BRANCH env-var, which is set automatically by
CircleCI. Annoyingly, this is set to the name of the source branch for a merge,
rather than the target branch. The behavior of this check for each branch type
is listed:
if CIRCLE_BRANCH == "master":
This CI run is the nightly CI run for the master branch.
Make sure all submodules of the master branch point to master
else if CIRCLE_BRANCH == "dev":
This CI run is most likely somebody doing a dev->master merge.
Make sure all submodules of the dev branch point to master
else:
This CI run is likely somebody merging a feature branch into dev.
Allow submodule pointers of the dev branch to point to master OR dev
