Add ability to read commit graph form mutable (gated by JK)

Summary:
# This stack
We want commit_graph blobs to be in mutable storage as they mutate and the current manifold caching has too long of a TTL.
In this stack I:
1. Start uploading commit graph blobs to mutable storage (without stopping writing to the current one)
2. Add a just knob to switch to read commit graph pfrom mutable, which I will enable after 1 and 2 have landed and commit graph blobs have been written to mutable
3. Clean up moving reads and writes to mutable

# This diff
Step 2

Reviewed By: RajivTS, YousefSalama

Differential Revision: D89664255

fbshipit-source-id: c3160c17d02ea5e39b938b25863bd99300930377

Author: Clara Rull

eden/mononoke/repo_factory/src/lib.rs

@@ -1923,12 +1923,18 @@ impl RepoFactory {
             Some(preloaded_commit_graph_key)
                 if !self.env.commit_graph_options.skip_preloading_commit_graph =>
             {
+                let blobstore = match justknobs::eval(
+                    "scm/mononoke:commit_graph_use_mutable_storage",
+                    None,
+                    None,
+                )? {
+                    true => &repo_config.storage_config.mutable_blobstore,
+                    false => &repo_config.storage_config.blobstore,
+                };
                 let blobstore_without_cache = self
                     .mutable_repo_blobstore_from_blobstore(
                         repo_identity,
-                        &self
-                            .blobstore_no_cache(&repo_config.storage_config.blobstore)
-                            .await?,
+                        &self.blobstore_no_cache(blobstore).await?,
                     )
                     .await?;
 

eden/mononoke/tools/admin/src/commands/commit_graph/update_preloaded.rs

@@ -120,12 +120,25 @@ pub(super) async fn update_preloaded(
         );
 
     let preloaded_edges = match args.rebuild {
-        false => match repo.repo_blobstore().get(ctx, &args.blobstore_key).await? {
-            Some(bytes) => {
-                preloaded_commit_graph_storage::deserialize_preloaded_edges(bytes.into_raw_bytes())?
+        false => {
+            #[cfg(fbcode_build)]
+            let bytes =
+                if justknobs::eval("scm/mononoke:commit_graph_use_mutable_storage", None, None)? {
+                    repo.mutable_repo_blobstore()
+                        .get(ctx, &args.blobstore_key)
+                        .await?
+                } else {
+                    repo.repo_blobstore().get(ctx, &args.blobstore_key).await?
+                };
+            #[cfg(not(fbcode_build))]
+            let bytes = repo.repo_blobstore().get(ctx, &args.blobstore_key).await?;
+            match bytes {
+                Some(bytes) => preloaded_commit_graph_storage::deserialize_preloaded_edges(
+                    bytes.into_raw_bytes(),
+                )?,
+                None => Default::default(),
             }
-            None => Default::default(),
-        },
+        }
         true => Default::default(),
     };