t: teach lint that RHS of 'local VAR=VAL' needs to be quoted

Teach t/check-non-portable-shell.pl that right hand side of the
assignment done with "local VAR=VAL" need to be quoted.  We
deliberately target only VAL that begins with $ so that we can catch

 - $variable_reference and positional parameter reference like $4
 - $(command substitution)
 - ${variable_reference-with_magic}

while excluding

 - $'\n' that is a bash-ism freely usable in t990[23]
 - $(( arithmetic )) whose result should be $IFS safe.
 - $? that also is $IFS safe

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster

t/check-non-portable-shell.pl

@@ -47,6 +47,8 @@ sub err {
 	/\bgrep\b.*--file\b/ and err 'grep --file FILE is not portable (use grep -f FILE)';
 	/\b[ef]grep\b/ and err 'egrep/fgrep obsolescent (use grep -E/-F)';
 	/\bexport\s+[A-Za-z0-9_]*=/ and err '"export FOO=bar" is not portable (use FOO=bar && export FOO)';
+	/\blocal\s+[A-Za-z0-9_]*=\$([A-Za-z0-9_{]|[(][^(])/ and
+		err q(quote "$val" in 'local var=$val');
 	/^\s*([A-Z0-9_]+=(\w*|(["']).*?\3)\s+)+(\w+)/ and exists($func{$4}) and
 		err '"FOO=bar shell_func" assignment extends beyond "shell_func"';
 	$line = '';