Allow viewing past commits in Zed (#27636)

This PR adds functionality for loading the diff for an arbitrary git
commit, and displaying it in a tab. To retrieve the diff for the commit,
I'm using a single `git cat-file --batch` invocation to efficiently load
both the old and new versions of each file that was changed in the
commit.

Todo

* Features
* [x] Open the commit view when clicking the most recent commit message
in the commit panel
  * [x] Open the commit view when clicking a SHA in a git blame column
  * [x] Open the commit view when clicking a SHA in a commit tooltip
  * [x] Make it work over RPC
  * [x] Allow buffer search in commit view
* [x] Command palette action to open the commit for the current blame
line
* Styling
* [x] Add a header that shows the author, timestamp, and the full commit
message
  * [x] Remove stage/unstage buttons in commit view
  * [x] Truncate the commit message in the tab
* Bugs
  * [x] Dedup commit tabs within a pane
  * [x] Add a tooltip to the tab

Release Notes:

- Added the ability to show past commits in Zed. You can view the most
recent commit by clicking its message in the commit panel. And when
viewing a git blame, you can show any commit by clicking its sha.

crates/collab/src/rpc.rs

@@ -413,6 +413,7 @@ impl Server {
             .add_request_handler(forward_mutating_project_request::<proto::GitInit>)
             .add_request_handler(forward_read_only_project_request::<proto::GetRemotes>)
             .add_request_handler(forward_read_only_project_request::<proto::GitShow>)
+            .add_request_handler(forward_read_only_project_request::<proto::LoadCommitDiff>)
             .add_request_handler(forward_read_only_project_request::<proto::GitReset>)
             .add_request_handler(forward_read_only_project_request::<proto::GitCheckoutFiles>)
             .add_request_handler(forward_mutating_project_request::<proto::SetIndexText>)