reftable/writer: fix index corruption when writing multiple indices

reftable/writer: fix index corruption when writing multiple indices

Each reftable may contain multiple types of blocks for refs, objects and
reflog records, where each of these may have an index that makes it more
efficient to find the records. It was observed that the index for log
records can become corrupted under certain circumstances, where the
first entry of the index points into the object index instead of to the
log records.

As it turns out, this corruption can occur whenever we write a log index
as well as at least one additional index. Writing records and their index
is basically a two-step process:

  1. We write all blocks for the corresponding record. Each block that
     gets written is added to a list of blocks to index.

  2. Once all blocks were written we finish the section. If at least two
     blocks have been added to the list of blocks to index then we will
     now write the index for those blocks and flush it, as well.

When we have a very large number of blocks then we may decide to write a
multi-level index, which is why we also keep track of the list of the
index blocks in the same way as we previously kept track of the blocks
to index.

Now when we have finished writing all index blocks we clear the index
and flush the last block to disk. This is done in the wrong order though
because flushing the block to disk will re-add it to the list of blocks
to be indexed. The result is that the next section we are about to write
will have an entry in the list of blocks to index that points to the
last block of the preceding section's index, which will corrupt the log
index.

Fix this corruption by clearing the index after having written the last
block.

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