Fix bug #8627: Command line switches to change branch do not appear to work

Fix bug #8627: Command line switches to change branch do not appear to work

Move the handling of branch-(de)activate(master) to Buffer. This code was moved to Bufferview in [3a03e71c/lyxgit] because a cursor was necessary to call Undo::recordUndoFullDocument(). However, it turns out that the undo code is already prepared to handle an empty cursor (and do nothing in this case).

Therefore we do that and move the branch code to Buffer where it belongs.

Note that there was a bug in the previous code that broke undo support: recordUndo should always be called _before_ doing any change.