t1512: ignore whitespaces in wc -l output

Some implementations of sed (e.g. MacOS X) have whitespaces in the
output of "wc -l" that reads from the standard input.  Ignore these
whitespaces by not quoting the command substitution to be compared
with the constant "16".

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