completion: Allow dash as the first character for __git_ps1

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 <info@christian-hammerl.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>