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)

assets/keymaps/default-linux.json

@@ -1080,7 +1080,8 @@
   {
     "context": "StashList || (StashList > Picker > Editor)",
     "bindings": {
-      "ctrl-shift-backspace": "stash_picker::DropStashItem"
+      "ctrl-shift-backspace": "stash_picker::DropStashItem",
+      "ctrl-shift-v": "stash_picker::ShowStashItem"
     }
   },
   {
@@ -1266,6 +1267,14 @@
       "ctrl-pagedown": "settings_editor::FocusNextFile"
     }
   },
+  {
+    "context": "StashDiff > Editor",
+    "bindings": {
+      "ctrl-space": "git::ApplyCurrentStash",
+      "ctrl-shift-space": "git::PopCurrentStash",
+      "ctrl-shift-backspace": "git::DropCurrentStash"
+    }
+  },
   {
     "context": "SettingsWindow > NavigationMenu",
     "use_key_equivalents": true,