parse-options: cast long name for OPTION_ALIAS

parse-options: cast long name for OPTION_ALIAS

We assign the long name for OPTION_ALIAS options to a non-constant value
field. We know that the variable will never be written to, but this will
cause warnings once we enable `-Wwrite-strings`.

Cast away the constness to be prepared for this change.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>