reftable: fix seeking to refs in reflog implementation

reftable: fix seeking to refs in reflog implementation

Small reftables omit the log index. Currently,
ReftableWriter#shouldHaveIndex does this if there is a single-block
log, but other writers could decide on different criteria.

In the case that the log index is missing, we have to linearly search
for the right block. It is never appropriate to use binary search on
blocks for log data, as the blocks are compressed and therefore
irregularly sized.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Change-Id: Id59874edf6bf45c7dec502d9465888e077ffe198