How to pipe bash completion suggestions?

Question

I am trying to save bash completion suggestions to use them. So, I am trying to simulate two tabs and pipe the result to something else. I am looking for something like:

    $ getCompletions git checkout
    master
    branch_1
    otherBranch
    $ getCompletions git checkout | grep '^ma'
    master

I am trying to find a solution for an arbitrary command or at least just git.