Implement "pmbootstrap pull"

From https://gitlab.com/postmarketOS/postmarketos/issues/11#pmbootstrap-pull, crossed out what will be implemented at a later stage (see detailed TODO at the bottom of that issue).

[...] provide a shorthand command for "git pull" in both pmaports.git and pmdevices.git. pmaports.git for now. This should check first, if the repository in question is on "master" or a channel branch, and only then run "git pull". Regular users will benefit most from this change, because then they don't need to familiarize with where the repositories are and how to update them - just run one command.

Issue created, because I'm working on this now. 👷