Add a shortcut for "git pull --ff-only" in all repositories cloned by
pmbootstrap (currently pmaports and aports_upstream, new pmdevices
repository coming soon).
'pmbootstrap pull' will only update the repositories, if:
* they are on an officially supported branch (e.g. master)
* the history is not conflicting (fast-forward is possible)
* the git workdirs are clean
Otherwise it shows the user a descriptive message about what to do. The
list of supported branches is only "master" right now, and will be
extended in later commits, so we can have a stable branch for pmaports
based on Alpine's releases. More about that in the project direction
2020 issue.
Closes: #1858
e04712a636