pmbootstrap pull: new action (!1848)

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

pmb/parse/arguments.py

@@ -552,6 +552,10 @@ def arguments():
                                  help="force even if the file seems to be"
                                       " invalid")
 
+    # Action: pull
+    sub.add_parser("pull", help="update all git repositories that pmbootstrap"
+                   " cloned (pmaports, etc.)")
+
     if argcomplete:
         argcomplete.autocomplete(parser, always_complete_options="long")