newvers.sh: Unbreak building in Git repositories.

Building the kernel in Git repositories when git-svn is not available and
the "help.autocorrect" Git parameter is enabled results in Git trying to
replace the "svn" command (it does not know) with "serve". As a result the
output of the "git server" command is appended to the value of the
environmental variable VERINFO, which causes the auto generated vers.c
file to contain invalid C syntax (missing newline escapes):

    #define "@(#)FreeBSD 12.0-ALPHA7  r000eversion 2
    0015agent=git/2.19.0
    000cls-refs
    0012fetch=shallow
    0012server-option
    0000=5e2272613fa(splash-vt)"
    #define VERSTR "FreeBSD 12.0-ALPHA7  r000eversion 2
    0015agent=git/2.19.0
    000cls-refs
    0012fetch=shallow
    0012server-option
    0000=5e2272613fa(splash-vt)\n"

Using `-c help.autocorrect=0` seems to be a good solution as it does not
modify user's environment. I am not sure, however, if we should use
programs (or Git commands), which we are not sure exist (we never check if
git-svn is available on the host), as there may be more unexpected
behaviors like this one.

Reviewed by:	eadler, emaste, krion
Approved by:	re (gjb), krion (mentor)
Sponsored by:	Bally Wulff Games & Entertainment GmbH
Differential Revision:	https://reviews.freebsd.org/D17271

sys/conf/newvers.sh

@@ -183,7 +183,7 @@ done
 if findvcs .git; then
 	for dir in /usr/bin /usr/local/bin; do
 		if [ -x "${dir}/git" ] ; then
-			git_cmd="${dir}/git --git-dir=${VCSDIR}"
+			git_cmd="${dir}/git -c help.autocorrect=0 --git-dir=${VCSDIR}"
 			break
 		fi
 	done