tree: add size information when logging

[jlebon]

It's informative to see how large an image is, and the repartition of
the data across the different repos. This could also help catch issues
early on when the expected repartition doesn't match what one would
expect.

Assisted-by: Claude Opus 4.6

[gemini-code-assist]

Code Review

This pull request adds size information to the logs, which is very helpful for understanding the composition of the built image. The changes are well-implemented and include relevant tests. I have two suggestions for improving maintainability and robustness:

1. In src/components/mod.rs, I suggest using a dedicated struct for repository statistics instead of a tuple to improve code clarity.
2. In src/utils.rs, I recommend using an external crate like humansize for formatting file sizes, which is a more robust and standard approach than a custom implementation.