Supporting tree structure instead of flat list in navigation pane

[winzamark123]

Hi!

Just wanted to pretext, that installed revdiff 2 or 3 days ago, really love the project! It was taking me a long time to find a terminal diff viewer that fit the agentic workflow I liked and revdiff is hitting that sweet spot. Anyway heres an issue i'd like to flag with a ready PR if the maintainers allow. I was going to set up a PR initially but read the CONTRIBUTING.md and realized I should put up an issue first in case it doesnt fit the product direction.

Problem

current files are shown as a flat list in the navigation pane. Coming from an IDE perspective it's a bit hard to read and understand. I opted for an option to the user if they want to show a tree instead of just a flat list! Again, fully configurable and set by the user. Clicking Enter or space on the folder will collapse them while keeping the same behavior of clicking on the file becomes reviewed (same as before).

tree-instead-of-flat.mp4

[umputun]

open to this, with some scoping. First a correction: the pane isn't really a flat list today, it already groups files by directory (a dir header with its files indented under it). So what you're adding is real nesting plus collapse/expand, which is fine, I just want us on the same page about the actual delta.

my main worry is indentation. The tree pane defaults to about 2/10 of the terminal width and names already get truncated to avoid wrapping. Nesting a level per path segment (app/ui/sidepane/foo.go) eats that budget fast and truncates harder, which is the opposite of what you want.

conditions for a merge:

• opt-in, default off (the current view stays the default)
• bounded indent: single-child chain compression (collapse app/ui/sidepane/ into one node) plus a depth cap
• collapse state has to survive the filter toggles (annotated-only, unreviewed-only), reload, and reconcile, and stay correct for mouse-click selection and the nav scrollbar. All of those rebuild the entries list, so collapse is new persistent state layered on top, wants tests for those interactions
• distinct collapse key, not space or enter (space is mark_reviewed, enter focuses the diff)

if the fork already covers the filter/reload/collapse cases, open the PR and I'll review.