Update genPrefetchDir to find changed files since last saved state revision
Summary:
D30407722 (https://github.com/facebook/hhvm/commit/
b3a139997f6758b3ac7ad2c8a8743d7dd2e208e0) added a hh_server command `hh --genPrefetchDir`, which generates a directory of decls for all the changed files in the current commit. This diff updates the command such that it finds all the changed files since the last saved state commit (which might tens or hundreds) and generate decls for all files changed since the saved state commit.
Reviewed By: bobrenjc93
Differential Revision:
D31133939
fbshipit-source-id:
e8dcf036656b5c9cd2201c535e4d798cacf1ecea