Move parse-options's skip_prefix() to git-compat-util.h

Move parse-options's skip_prefix() to git-compat-util.h

builtin-remote.c and parse-options.c both have a skip_prefix() function,
for the same purpose. Move parse-options's one to git-compat-util.h and
let builtin-remote use it as well.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>