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

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>