reftable/stack: refactor reloading to use file descriptor

reftable/stack: refactor reloading to use file descriptor

We're about to introduce a stat(3P)-based caching mechanism to reload
the list of stacks only when it has changed. In order to avoid race
conditions this requires us to have a file descriptor available that we
can use to call fstat(3P) on.

Prepare for this by converting the code to use `fd_read_lines()` so that
we have the file descriptor readily available.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>