newvers.sh: speed up failing git-svn revision search

In the case of running newvers.sh on a git tree w/o git-svn-id notes we
previously piped the entire 'git log' to grep. Add --grep to the log
invocation to avoid processing log entries of no interest.

This saves about 2-3 seconds of newvers.sh run time on my SSD laptop.
Later changes will bring further speedups.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation