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