newvers: Set explicit git revision length

The --short flag is configurable. Setting an explicit length supports
reproducible builds.

Signed-off-by: Pat Maddox <pat@patmaddox.com>
Reviewed by: emaste, imp
Differential Revision: https://github.com/freebsd/freebsd-src/pull/1547

(cherry picked from commit a1097094c4c5d810287aca092f4ab5f9f86a426a)