makechrootpkg: Look harder for -R argument

We shouldn't be in the business of reparsing makepkg's arguments, but
since we have to treat the case of repackaging separately, do a better
job of trying to find signs of it happening. This change lets you pass
the longopt, --repackage, or multiple shortopts such as -RA, and still
get the intended effect.

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>