completion: Allow dash as the first character for __git_ps1
If the argument for `__git_ps1` begins with a dash, `printf` tries to interpret it as an option which results in an error message. The problem is solved by adding '--' before the argument to tell `printf` to not interpret the following argument as an option. Adding '--' directly to the argument does not help because the argument is enclosed by double quotes.
Signed-off-by: Christian Hammerl <email@example.com> Signed-off-by: Junio C Hamano <firstname.lastname@example.org>