[gtksourceview] Non-fast-forward update to branch wip/chergert/gsv-gtk4

[Christian Hergert <chergert src gnome org>]

The branch 'wip/chergert/gsv-gtk4' was changed in a way that was not a fast-forward update.
NOTE: This may cause problems for people pulling from the branch. For more information,
please see:

 https://wiki.gnome.org/Git/Help/NonFastForward

Commits removed from the branch:

  4b26808... completion: port most of test completion

Commits added to the branch:

  395ada9... completion: port to new completion provider interface