Summary:
In following diffs we will use scs to prefetch meta-data for files, so that this data
will be available with out fetching the file content (which will improve build times
on eden).
This builds up the proxy hash index that serves as a conversion between eden
specific identifiers and commit and path which we will use to index into scs.
Reviewed By: chadaustin
Differential Revision: D21820909
fbshipit-source-id: 17891f6772f49c7c183061d7a4df2fe0a3be9d25
264d749d67