reftable/reader: add comments to `table_iter_next()`

reftable/reader: add comments to `table_iter_next()`

While working on the optimizations in the preceding patches I stumbled
upon `table_iter_next()` multiple times. It is quite easy to miss the
fact that we don't call `table_iter_next_in_block()` twice, but that the
second call is in fact `table_iter_next_block()`.

Add comments to explain what exactly is going on here to make things
more obvious. While at it, touch up the code to conform to our code
style better.

Note that one of the refactorings merges two conditional blocks into
one. Before, we had the following code:

```
err = table_iter_next_block(&next, ti);
if (err != 0) {
	ti->is_finished = 1;
}
table_iter_block_done(ti);
if (err != 0) {
	return err;
}
```

As `table_iter_block_done()` does not care about `is_finished`, the
conditional blocks can be merged into one block:

```
err = table_iter_next_block(&next, ti);
table_iter_block_done(ti);
if (err != 0) {
	ti->is_finished = 1;
	return err;
}
```

This is both easier to reason about and more performant because we have
one branch less.

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