reftable/stack: stop using `read_in_full()`

reftable/stack: stop using `read_in_full()`

There is a single callsite of `read_in_full()` in the reftable library.
Open-code the function to reduce our dependency on the Git library.

Note that we only partially port over the logic from `read_in_full()`
and its underlying `xread()` helper. Most importantly, the latter also
knows to handle `EWOULDBLOCK` via `handle_nonblock()`. This logic is
irrelevant for us though because the reftable library never sets the
`O_NONBLOCK` option in the first place.

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