reftable/block: fix binary search over restart counter

Records store their keys prefix-compressed. As many records will share a common prefix (e.g. "refs/heads/"), this can end up saving quite a bit of disk space. The downside of this is that it is not possible to just seek into the middle of a block and consume the corresponding record because it may depend on prefixes read from preceding records.

To help with this usecase, the reftable format writes every n'th record without using prefix compression, which is called a "restart". The list of restarts is stored at the end of each block so that a reader can figure out entry points at which to read a full record without having to read all preceding records.

This allows us to do a binary search over the records in a block when searching for a particular key by iterating through the restarts until we have found the section in which our record must be located. From thereon we perform a linear search to locate the desired record.

This mechanism is broken though. In block_reader_seek() we call binsearch() over the count of restarts in the current block. The function we pass to compare records with each other computes the key at the current index and then compares it to our search key by calling strbuf_cmp(), returning its result directly. But binsearch() expects us to return a truish value that indicates whether the current index is smaller than the searched-for key. And unless our key exactly matches the value at the restart counter we always end up returning a truish value.

The consequence is that binsearch() essentially always returns 0, indicacting to us that we must start searching right at the beginning of the block. This works by chance because we now always do a linear scan from the start of the block, and thus we would still end up finding the desired record. But needless to say, this makes the optimization quite useless.

Fix this bug by returning whether the current key is smaller than the searched key. As the current behaviour was correct it is not possible to write a test. Furthermore it is also not really possible to demonstrate in a benchmark that this fix speeds up seeking records.

This may cause the reader to question whether this binary search makes sense in the first place if it doesn't even help with performance. But it would end up helping if we were to read a reftable with a much larger block size. Blocks can be up to 16MB in size, in which case it will become much more important to avoid the linear scan. We are not yet ready to read or write such larger blocks though, so we have to live without a benchmark demonstrating this.

Closes #278 (closed)