master from https://github.com/j6t/gitk
gitk: Add user preference to hide custom references
External tools such as Jujutsu may add many references that are of no
interest to the user. This preference hides them.
The non-custom refs are those that pass git's default decoration filter
(see set_default_decoration_filter).
The preference is off by default, maintaining current behavior.
Signed-off-by: Ori Avtalion <ori@avtalion.name>