newvers.sh: add support for gitup(1)

gitup writes a .gituprevision file into the shallow clone directory. Read that
file and print commit information only.

Submitted by: Michael Osipov <michael.osipov@siemens.com>
Pull Request: https://github.com/freebsd/freebsd-src/pull/449

While here, drop the redundant branch name from the git output and don't
count commits in shallow clones.

Reported by: Michael Osipov <michael.osipov@siemens.com>
MFC after: 2 weeks