git: Add diff view for stash entries (#38280)

Continues the work from #35927 to add a git diff view for stash entries.

[Screencast From 2025-09-17
19-46-01.webm](https://github.com/user-attachments/assets/ded33782-adef-4696-8e34-3665911c09c7)

Stash entries are [represented as
commits](https://git-scm.com/docs/git-stash#_discussion) except they
have up to 3 parents:

```
       .----W (this is the stash entry)
      /    /|
-----H----I |
           \|
            U
```

Where `H` is the `HEAD` commit, `I` is a commit that records the state
of the index, and `U` is another commit that records untracked files
(when using `git stash -u`).

Given this, I modified the existing commit view struct to allow loading
stash and commits entries with git sha identifier so that we can get a
similar git diff view for both of them.

The stash diff is generated by comparing the stash commit with its
parent (`<commit>^` or `H` in the diagram) which generates the same diff
as doing `git stash show -p <stash entry>`. This *can* be
counter-intuitive since a user may expect the comparison to be made
between the stash commit and the current commit (`HEAD`), but given that
the default behavior in git cli is to compare with the stash parent, I
went for that approach.

Hoping to get some feedback from a Zed team member to see if they agree
with this approach.

Release Notes:

- Add git diff view for stash entries
- Add toolbar on git diff view for stash entries
- Prompt before executing a destructive stash action on diff view
- Fix commit view for merge commits  (see #38289)

crates/zed/src/zed.rs

@@ -25,6 +25,7 @@ use feature_flags::{FeatureFlagAppExt, PanicFeatureFlag};
 use fs::Fs;
 use futures::future::Either;
 use futures::{StreamExt, channel::mpsc, select_biased};
+use git_ui::commit_view::CommitViewToolbar;
 use git_ui::git_panel::GitPanel;
 use git_ui::project_diff::ProjectDiffToolbar;
 use gpui::{
@@ -1049,6 +1050,8 @@ fn initialize_pane(
             toolbar.add_item(migration_banner, window, cx);
             let project_diff_toolbar = cx.new(|cx| ProjectDiffToolbar::new(workspace, cx));
             toolbar.add_item(project_diff_toolbar, window, cx);
+            let commit_view_toolbar = cx.new(|cx| CommitViewToolbar::new(workspace, cx));
+            toolbar.add_item(commit_view_toolbar, window, cx);
             let agent_diff_toolbar = cx.new(AgentDiffToolbar::new);
             toolbar.add_item(agent_diff_toolbar, window, cx);
             let basedpyright_banner = cx.new(|cx| BasedPyrightBanner::new(workspace, cx));