ssh: add command to push tag to FREEBSD-upgrade instructions

Because it appears git push --follow-tags may push extra, undesired
tags document both techniques (pushing the specific vendor/openssh/X.YpZ
tag and pushing all with --follow-tags, using --dry-run first).

Discussed with: imp, lwhsu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33605

(cherry picked from commit ab7d09596958791d20596b16b81a957495e4b370)