add comment explaining why fetch-tree-aux-data is enabled

Summary:
# Context

The new AugmentedManifest format used by Sapling and Mononoke will support looking up Trees/Blobs from CAS via (Blake3,size) pairs. We already provide a way for users to get (Blake3,size) pairs for blobs, but there is currently no easy way to get this information for trees.

We want to add support to getAttributesFromFilesV2 for looking up the (Blake3,size) pairs for trees as well. To do this, we'll need to add a method for querying TreeMetadata (including size and Blake3 hashes) via the SaplingBackingStore.

This initial implementation of TreeMetadata will only include the following features:

1) Ability to query TreeMetadata from the SaplingBackingStore via a getTreeMetadata endpoint
2) The ability to request TreeMetadata from the getAttributesFromFilesV2 thrift endpoint

In the future, we will extend the implementation to support:

1) Automatically fetching TreeMetadata during BackingStore::getTree requests
2) Caching TreeMetadata in Eden's in-memory caches

# This diff

We should document why certain configs are turned on. This diff documents one config I recently enabled

Reviewed By: muirdm

Differential Revision: D59662771

fbshipit-source-id: 411798260aebf7f2828cf63994a1029dbee15130

Author: MichaelCuevas

eden/integration/lib/hgrepo.py

@@ -292,8 +292,11 @@ def init(
         hgrc["remotefilelog"]["server"] = "false"
         hgrc["remotefilelog"]["reponame"] = "test"
         hgrc["remotefilelog"]["cachepath"] = cachepath
+
+        # We should allow fetching tree aux data along with trees
         hgrc.add_section("scmstore")
         hgrc["scmstore"]["fetch-tree-aux-data"] = "true"
+
         # Some tests set these configs on their own. We shouldn't overwrite them.
         if not hgrc.has_section("paths"):
             hgrc.add_section("paths")