git-gui: Fix gitk for branch whose name matches local file

When trying to run gitk on a branch name whose name matches a local file, it will toss an error saying that the name is ambiguous. Adding a pair of dashes will make gitk parse the options to the left of it as branch names. Since wish eats the first pair of dashes we throw at it, we need to add a second one to ensure they get through. Signed-off-by: Peter Krefting <peter@softwolves.pp.se> Signed-off-by: Shawn O. Pearce <spearce@spearce.org>