store: don't include cached objects in Debug format

I was looking into some overly verbose logs and happened to notice
that `Store` uses the default derived, which presumably means it's
going to include all objects in its cached. Just including the
backend's debug string seems enough.

lib/src/store.rs

@@ -16,6 +16,7 @@
 
 use std::any::Any;
 use std::collections::HashMap;
+use std::fmt::{Debug, Formatter};
 use std::io::Read;
 use std::sync::{Arc, RwLock};
 
@@ -33,7 +34,6 @@ use crate::tree_builder::TreeBuilder;
 
 /// Wraps the low-level backend and makes it return more convenient types. Also
 /// adds caching.
-#[derive(Debug)]
 pub struct Store {
     backend: Box<dyn Backend>,
     commit_cache: RwLock<HashMap<CommitId, Arc<backend::Commit>>>,
@@ -41,6 +41,14 @@ pub struct Store {
     use_tree_conflict_format: bool,
 }
 
+impl Debug for Store {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), std::fmt::Error> {
+        f.debug_struct("Store")
+            .field("backend", &self.backend)
+            .finish_non_exhaustive()
+    }
+}
+
 impl Store {
     pub fn new(backend: Box<dyn Backend>, use_tree_conflict_format: bool) -> Arc<Self> {
         Arc::new(Store {