backingstore: avoid constantly reloading sl repo

Summary:
When there `make_matcher` callback returned `None` we were callign `make_matcher` every time through the loop. The callback loads the Sapling Repo object, which is too heavy to do every time.

Fix by tweaking things so `make_matcher` is only invoked when the current commit changes.

Reviewed By: Alramech

Differential Revision: D88696778

fbshipit-source-id: 1dbf987ded3aae608b9d3cbc68e9cf0e5f5dc028

Author: muirdm

eden/scm/lib/backingstore/src/prefetch.rs

@@ -138,10 +138,7 @@ pub(crate) fn prefetch_manager(
                 }
             };
 
-            if prev_commit_id
-                .as_ref()
-                .is_some_and(|id| *id != current_commit_id)
-            {
+            if prev_commit_id.as_ref() != Some(&current_commit_id) {
                 tracing::info!(
                     ?prev_commit_id,
                     ?current_commit_id,
@@ -153,20 +150,17 @@ pub(crate) fn prefetch_manager(
                 handled_prefetches.clear();
                 batch_dir_prefetches.clear();
                 current_manifest.take();
-                sparse_matcher.take();
-            }
 
-            prev_commit_id = Some(current_commit_id);
-
-            // Reevaluate the sparse matcher since it can change across commits
-            if sparse_matcher.is_none() {
+                // Reevaluate the sparse matcher since it can change across commits
                 sparse_matcher = match (make_matcher)(repo_root.clone()) {
                     Ok(m) => m,
                     Err(e) => {
                         tracing::error!("fetching sparse matcher for workingcopy: {:?}", e);
                         continue;
                     }
                 };
+
+                prev_commit_id = Some(current_commit_id);
             }
 
             // Currently active walks according to the walk detector. Note that it will only report
@@ -1070,7 +1064,7 @@ mod test {
 
         // Wait for prefetch to finish.
         for _ in 0..10 {
-            if store.key_fetch_count() < 4 {
+            if store.key_fetch_count() < 7 {
                 std::thread::sleep(Duration::from_millis(1));
             }
         }