More Git panel refinements (#21928)

- Add and wire through git method stubs
- Organize render methods
- Track modifier changes
- Swap commit buttons when `option`/`alt` is held
- More TODOs

Release Notes:

- N/A

crates/git_ui/TODO.md

@@ -0,0 +1,42 @@
+### General
+
+- [x] Disable staging and committing actions for read-only projects
+
+### List
+
+- [ ] Git status item
+- [ ] Directory item
+- [ ] Scrollbar
+- [ ] Add indent size setting
+- [ ] Add tree settings
+
+### List Items
+
+- [ ] Context menu
+  - [ ] Discard Changes
+  - ---
+  - [ ] Ignore
+  - [ ] Ignore directory
+  - ---
+  - [ ] Copy path
+  - [ ] Copy relative path
+  - ---
+  - [ ] Reveal in Finder
+
+### Commit Editor
+
+- [ ] Add commit editor
+- [ ] Add commit message placeholder & add commit message to store
+- [ ] Add a way to get the current collaborators & automatically add them to the commit message as co-authors
+- [ ] Add action to clear commit message
+- [x] Swap commit button between "Commit" and "Commit All" based on modifier key
+
+### Component Updates
+
+- [ ] ChangedLineCount (new)
+  - takes `lines_added: usize, lines_removed: usize`, returns a added/removed badge
+- [ ] GitStatusIcon (new)
+- [ ] Checkbox
+  - update checkbox design
+- [ ] ScrollIndicator
+  - shows a gradient overlay when more content is available to be scrolled