Add a concurrent tree-diffing algo

cli/examples/custom-backend/main.rs

@@ -117,6 +117,10 @@ impl Backend for JitBackend {
         self.inner.empty_tree_id()
     }
 
+    fn concurrency(&self) -> usize {
+        1
+    }
+
     async fn read_file(&self, path: &RepoPath, id: &FileId) -> BackendResult<Box<dyn Read>> {
         self.inner.read_file(path, id).await
     }

lib/src/backend.rs

@@ -473,6 +473,15 @@ pub trait Backend: Send + Sync + Debug {
 
     fn empty_tree_id(&self) -> &TreeId;
 
+    /// An estimate of how many concurrent requests this backend handles well. A
+    /// local backend like the Git backend (at until it supports partial clones)
+    /// may want to set this to 1. A cloud-backed backend may want to set it to
+    /// 100 or so.
+    ///
+    /// It is not guaranteed that at most this number of concurrent requests are
+    /// sent.
+    fn concurrency(&self) -> usize;
+
     async fn read_file(&self, path: &RepoPath, id: &FileId) -> BackendResult<Box<dyn Read>>;
 
     fn write_file(&self, path: &RepoPath, contents: &mut dyn Read) -> BackendResult<FileId>;

lib/src/git_backend.rs

@@ -612,6 +612,10 @@ impl Backend for GitBackend {
         &self.empty_tree_id
     }
 
+    fn concurrency(&self) -> usize {
+        1
+    }
+
     async fn read_file(&self, _path: &RepoPath, id: &FileId) -> BackendResult<Box<dyn Read>> {
         self.read_file_sync(id)
     }

lib/src/local_backend.rs

@@ -149,6 +149,10 @@ impl Backend for LocalBackend {
         &self.empty_tree_id
     }
 
+    fn concurrency(&self) -> usize {
+        1
+    }
+
     async fn read_file(&self, _path: &RepoPath, id: &FileId) -> BackendResult<Box<dyn Read>> {
         let path = self.file_path(id);
         let file = File::open(path).map_err(|err| map_not_found_err(err, id))?;