Follow up to https://github.com/zed-industries/zed/pull/59884

This PR refines how the git panel behaves when grouping changes by
staging state, with a focus on making staging workflows predictable and
conflict resolution safe.

- Renamed the "Group By" menu options to describe what you actually see:
**"Tracked & Untracked"** (was "Status") and **"Staged & Unstaged"**
(was "Staging").
- When grouping by staged & unstaged, both sections now stay visible
even when empty, showing a placeholder message ("No staged changes yet"
/ "No unstaged changes"). Previously an empty section disappeared
entirely, which made the panel layout jump around as you staged and
unstaged files, and made it harder to tell at a glance that nothing was
staged yet.
- Section header controls are now consistent checkboxes everywhere
(previously a mix of checkboxes and +/− icon buttons), with "Stage All"
/ "Unstage All" tooltips. Headers of empty sections render no checkbox
and don't react to clicks or hover. I appreciate the debate that
happened in the PR linked above about this but I was personally having a
hard time understanding what was the difference in interaction given the
action was exactly the same, we were just having different UIs, which
looked inconsistent.
- Arrow-key navigation now skips over section headers and empty-section
placeholder rows in both flat and tree view, instead of getting stuck or
selecting non-interactive rows — including when jumping to the first or
last entry.
- In the staged & unstaged grouping, a partially staged file appears in
both sections. Each row's checkbox and tooltip now follow the section
it's rendered in: rows in Staged always unstage, rows in Unstaged always
stage — including via shift-click range operations, which now also
support bulk *unstaging* within the Staged section (previously ranges
could only stage). The context menu's Stage/Unstage label follows the
same rule and stays in sync with in-flight staging operations.
- Conflicted files are grouped by whether the current merge marked them
conflicted (rather than raw status), so a conflict you've resolved stays
visible under "Conflicts" until the merge concludes. On top of that,
resolution is now one-way in the UI:
- Ticking a conflicted file's checkbox marks it resolved (stages it).
Once resolved, the checkbox is disabled with a "Conflict marked as
resolved" tooltip — unticking it would silently discard git's record of
the unmerged base/ours/theirs versions, a round-trip git can't actually
perform.
- The same lock applies everywhere the file can be reached: the keyboard
toggle, folder checkboxes in tree view, the Conflicts section header
(disabled once all conflicts are resolved), "Stage All"/"Unstage All" on
the Staged/Unstaged headers, and shift-click range sweeps — none of them
will resolve or un-resolve a conflict as a side effect.
- The explicit `git: unstage file` action still works as a deliberate
escape hatch.

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Here's a video, where you can see I stage and unstage whole files, make
a partial staging, and get into a merge-conflict state, where the
conflicted files are tagged as resolved:


https://github.com/user-attachments/assets/d27ef9c6-5691-45c1-91ab-c3b152aaaa60

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Release Notes:

- N/A _(given the feature hasn't been released yet_)