[hack] Watchman.get_all_files should update clockspec
[hack] Watchman.get_all_files should update clockspec

Summary:Otherwise we may get more file change notifications than necessary. I think
this happens when watchman just starts indexing a directory when we first call
it. E.g. for a newly created directory:

1. hh_server calls `watchman clock`. watchman returns the current
  clockspec t1 and then goes about indexing the directory.
2. hh_server calls `watchman query`. watchman returns all the files in that
  directory, as well as the clockspec t2. Before this diff, we were ignoring t2.
3. hh_server calls `watchman since` and asks for the changes since t1. Since
  watchman only started indexing after t1, it returns the full list of files.

Reviewed By: ericlippert

Differential Revision: D3117133

fb-gh-sync-id: 99d317546dfa222884f7a49dea2c1da624429df0
fbshipit-source-id: 99d317546dfa222884f7a49dea2c1da624429df0

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