Move "async move" a few characters to the left in cx.spawn() (#26758)

This is the core change:
https://github.com/zed-industries/zed/pull/26758/files#diff-044302c0d57147af17e68a0009fee3e8dcdfb4f32c27a915e70cfa80e987f765R1052

TODO:
- [x] Use AsyncFn instead of Fn() -> Future in GPUI spawn methods
- [x] Implement it in the whole app
- [x] Implement it in the debugger 
- [x] Glance at the RPC crate, and see if those box future methods can
be switched over. Answer: It can't directly, as you can't make an
AsyncFn* into a trait object. There's ways around that, but they're all
more complex than just keeping the code as is.
- [ ] Fix platform specific code

Release Notes:

- N/A

crates/git/src/fake_repository.rs

@@ -215,7 +215,7 @@ impl GitRepository for FakeGitRepository {
         &self,
         path: RepoPath,
         _content: Rope,
-        _cx: AsyncApp,
+        _cx: &mut AsyncApp,
     ) -> BoxFuture<Result<crate::blame::Blame>> {
         let state = self.state.lock();
         let result = state

crates/git/src/repository.rs

@@ -207,7 +207,7 @@ pub trait GitRepository: Send + Sync {
         &self,
         path: RepoPath,
         content: Rope,
-        cx: AsyncApp,
+        cx: &mut AsyncApp,
     ) -> BoxFuture<Result<crate::blame::Blame>>;
 
     /// Returns the absolute path to the repository. For worktrees, this will be the path to the
@@ -678,7 +678,7 @@ impl GitRepository for RealGitRepository {
         &self,
         path: RepoPath,
         content: Rope,
-        cx: AsyncApp,
+        cx: &mut AsyncApp,
     ) -> BoxFuture<Result<crate::blame::Blame>> {
         let working_directory = self.working_directory();
         let git_binary_path = self.git_binary_path.clone();