Let the chunker return the content of the chunks

[bchapuis]

ATM, the chunker only returns the breakpoints found in a stream of data.

type Chunk struct {
    Start  uint
    Length uint
    Cut    uint64
    Digest []byte
}

This is fine for chunking files which are read a first time for finding chunks and a second time for persisting chunks. However, this is not appropriate when dealing with stream coming from stdin such as mysql dumps. In that case, the chunker should return chunks that contains the actual bytes of the chunk.

type Chunk struct {
    Start  uint
    Length uint
    Cut    uint64
    Digest []byte
    Content []byte
}

[fd0]

This bit me today: I've discovered that restic did not backup some files correctly because they changed during the backup (and the checking code detected it correctly). We should implement this.

[fd0]

Addressed in 7ae4eba