gitk: New option to hide remote refs

gitk: New option to hide remote refs

In repositories with lots of remotes, looking at the history in gitk
can be borderline insane with all the red labels for remote refs.
Introduce a new option in the preferences that makes gitk ignore
remote refs entirely, so they don't take up space in the display.

Wished-for-by: Thell Fowler <tbfowler4@gmail.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Paul Mackerras <paulus@samba.org>