reftable: check for trailing newline in 'tables.list'

reftable: check for trailing newline in 'tables.list'

In the reftable format, the 'tables.list' file contains a
newline separated list of tables. While we parse this file, we do not
check or care about the last newline. Tighten the parser in
`parse_names()` to return an appropriate error if the last newline is
missing.

This requires modification to `parse_names()` to now return the error
while accepting the output as a third argument.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>