Add --preserve-file-hash-cache option to `buck perf rk` command
Add --preserve-file-hash-cache option to `buck perf rk` command

Summary:
Even when faking the hashes and other filesystem operations, there's
still some non-insignificant work that is done to fill the cache. To
better reflect performance of incremental builds, this option will keep
the file hash cache between runs.

Reviewed By: jtorkkola

fbshipit-source-id: 383b8c75cd

facebook/buck,facebook/buck,Addepar/buck,JoelMarcey/buck,kageiit/buck,facebook/buck,JoelMarcey/buck,facebook/buck,nguyentruongtho/buck,zpao/buck,JoelMarcey/buck,JoelMarcey/buck,nguyentruongtho/buck,JoelMarcey/buck,facebook/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,kageiit/buck,zpao/buck,facebook/buck,zpao/buck,Addepar/buck,zpao/buck,kageiit/buck,Addepar/buck,kageiit/buck,JoelMarcey/buck,nguyentruongtho/buck,Addepar/buck,JoelMarcey/buck,JoelMarcey/buck,nguyentruongtho/buck,Addepar/buck,JoelMarcey/buck,Addepar/buck,Addepar/buck,Addepar/buck,nguyentruongtho/buck,nguyentruongtho/buck,kageiit/buck,zpao/buck,kageiit/buck,facebook/buck,JoelMarcey/buck,Addepar/buck,nguyentruongtho/buck,zpao/buck,Addepar/buck,Addepar/buck,Addepar/buck,Addepar/buck,zpao/buck,kageiit/buck