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

sys/conf/newvers.sh

@@ -218,8 +218,9 @@ if [ -n "$git_cmd" ] ; then
 		svn=" r${svn}"
 		git="=${git}"
 	else
-		svn=`$git_cmd log | grep '^    git-svn-id:' | head -1 | \
-		     sed -n 's/^.*@\([0-9][0-9]*\).*$/\1/p'`
+		svn=`$git_cmd log --grep '^git-svn-id:' | \
+		    grep '^    git-svn-id:' | head -1 | \
+		    sed -n 's/^.*@\([0-9][0-9]*\).*$/\1/p'`
 		if [ -z "$svn" ] ; then
 			svn=`$git_cmd log --format='format:%N' | \
 			     grep '^svn ' | head -1 | \