git: Add diff view for stash entries

[AlvaroParker]

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

Screencast.From.2025-09-17.19-46-01.webm

Stash entries are represented as commits 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 Merge commits aren't loading diff files on commit view #38289)

[AlvaroParker]

Hey @cole-miller, sorry to ping you here! Just wanted to check if there's anything I can change or improve to help move this along, let me know. Thanks, and sorry again for the ping :)

(same goes for #38719)

[cole-miller]

Thanks @AlvaroParker and sorry for not reviewing this sooner--the code and design here generally look great, well done! I really like how this reuses the existing commit view and even simplifies it a little bit in the process. I just have a couple of small design comments and then this should be good to merge.

[cole-miller]

Thanks @AlvaroParker, nicely done!