remove prefix argument from pathspec_prefix

remove prefix argument from pathspec_prefix

Passing a prefix to a function that is supposed to find the prefix is
strange. And it's really only used if the pathspec is NULL. Make the
callers handle this case instead.

As we are always returning a fresh copy of a string (or NULL), change the
type of the returned value to non-const "char *".

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>