add justknob for disabling megarepo mutable renames

Summary: Mutable renames created by megarepo_api are reaching scaling limits.  Add a knob that lets us stop creating them.

Reviewed By: andreacampi

Differential Revision: D65817987

fbshipit-source-id: f6a76ae317bf14577585ff6bb5c417d3786acbce

eden/mononoke/megarepo_api/src/common.rs

@@ -463,16 +463,24 @@ pub trait MegarepoOp<R> {
         )
         .freeze()?;
 
-        let mutable_renames = self
-            .create_mutable_renames(
+        let mutable_renames = if let Ok(true) = justknobs::eval(
+            "scm/mononoke:megarepo_disable_mutable_rename_creation",
+            None,
+            Some(repo.repo_identity().name()),
+        ) {
+            // Skip creating mutable renames
+            Vec::new()
+        } else {
+            self.create_mutable_renames(
                 ctx,
                 repo,
                 cs_id,
                 moved_bcs.get_changeset_id(),
                 mover,
                 directory_mover,
             )
-            .await?;
+            .await?
+        };
 
         let source_and_moved_changeset = SourceAndMovedChangesets {
             source: cs_id,

eden/mononoke/mononoke_macros/just_knobs_defaults/just_knobs.json

@@ -41,7 +41,8 @@
     "scm/mononoke:remote_gitimport_derive_ccsm": true,
     "scm/mononoke:changeset_path_context_use_skeleton_manifest_v2": true,
     "scm/mononoke:case_conflicts_check_use_ccsm": true,
-    "scm/mononoke:should_set_committer_info_to_author_info_if_empty": true
+    "scm/mononoke:should_set_committer_info_to_author_info_if_empty": true,
+    "scm/mononoke:megarepo_disable_mutable_rename_creation": false
   },
   "ints": {
     "scm/mononoke_timeouts:repo_client_clone_timeout_secs": 14400,