Introduce --diffable flag

[jart]

Sometimes it's useful to get an overview of how tensors changes when using different quantization formats. For example:

diff -u <(gguf-tools show --diffable ggml-model-bf16.gguf)
<(gguf-tools show --diffable ggml-model-Q6_K.gguf) | less

Is now able to produces nice clean output. Without this change, every line would have been different due to the file offsets and byte sizes which means diff -u would produce one gigantic unreadable chunk.