cmd/image/tree: refactor

[laurazard]

- What I did

Just some refactors to the image tree formatting logic to make reuse/reading easier. Consolidates the printing in one place, and reduces how far down we need to pass our writer.

This also makes testing these functions less involved.

- How I did it

(wip, I need to push more commits)

Make most of the formatting functions return strings, instead of printing directly.

- How to verify it

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 26 lines in your changes missing coverage. Please review.

Project coverage is 59.60%. Comparing base (da9e984) to head (4f849b4).
Report is 34 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5572      +/-   ##
==========================================
+ Coverage   59.15%   59.60%   +0.44%     
==========================================
  Files         342      345       +3     
  Lines       29079    29102      +23     
==========================================
+ Hits        17203    17346     +143     
+ Misses      10901    10787     -114     
+ Partials      975      969       -6     

[thaJeztah]

Slightly on the fence on this, because it means that we'd now be buffering all output in a string, instead sending it to the writer 🤔

[laurazard]

I didn't think that was a big deal for this output, but I can change it back. This isn't really in the hotpath, and makes reuse easier. Otherwise, if we want to reuse this (in the push output for example) and need to get this output to reformat it, we'll need to actually create a buffer, call this, let it write, parse the buffer and then reformat it and write to stdout.

[laurazard]

Thinking about it, I think I can do something a little nicer with the other refactors I haven't pushed yet. One sec.