When merging a passing PR which was not on top of the last commit of the
main branch, we may break CI.  This make sure a CI run is triggered when
the main branch is updated.  As GitHub kindly display the status of the
last commit on a project page, this allow to quickly see if a module CI
works as intended or not.

The main branch of most of our modules is `master` but a few use the
more inclusive `main` name, so enable ci for both.

Some consumers of modulesync want to enable this for e.g. a `develop`
branch, so make it tunable and use the `main` and `master` names as
default value for this new setting which should work out-of the box for
most cases.