bundle: split out a helper function to create pack data
The create_bundle() function, while it does one single logical
thing, takes a rather large implementation to do so.
Let's start separating what it does into smaller steps to make it
easier to see what is going on. This is a first step to separate
out the actual pack-data generation, after the earlier part of the
function figures out which part of the history to place in the
bundle.