Tweaks to libarchive's upgrade instructions

I would like to use this file as an example of the FREEBSD-upgrade
convention, see D42302. libarchive is picked somewhat arbitrarily as a
longstanding piece of contrib software in FreeBSD.

• Remove SVN references (HEAD/trunk)
• Mention the vendor/libarchive git branch
• Update link to import instructions
• Remove $FreeBSD$

Reviewed by: mm, imp, emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42308