newvers: append commit count to uname version string

In a git world this provides a facsimile of a monotonically increasing
version number.  This might be refined further, but this provides a
starting point for investigation.

Reviewed by:	cem
Relnotes:	Yes
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D20462

sys/conf/newvers.sh

@@ -291,6 +291,10 @@ if [ -n "$git_cmd" ] ; then
 			git=" ${git}"
 		fi
 	fi
+	git_cnt=$($git_cmd rev-list --count HEAD 2>/dev/null)
+	if [ -n "$git_cnt" ] ; then
+		git="${git}-c${git_cnt}"
+	fi
 	git_b=$($git_cmd rev-parse --abbrev-ref HEAD)
 	if [ -n "$git_b" ] ; then
 		git="${git}(${git_b})"