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

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 <redacted>
Signed-off-by: Shawn O. Pearce <redacted>