Fix ForeignAPIRepo::fileExistsBatch()

Fix ForeignAPIRepo::fileExistsBatch()

ForeignAPIRepo::fileExistsBatch() is extremely broken:
* If passed an array with numeric keys (e.g. as called from
  FileRepo::fileExists()), it will cache the fact that the item at index
  0 from the first call was found or not found, no matter what the
  actual file being queried is!
* But it doesn't even use its cache correctly. Rather than checking the
  value cached, it just assumes anything that was cached exists.
* It checks whether the page in the remote repository has a description
  page, not whether a file actually exists for the title.
* But it doesn't do that right either, so in the vast majority of cases
  it will decide everything exists.

All of this is offset by the fact that I can't find any code path that
would actually call this function.

Change-Id: I421e8892b642f5267b8dd755183ebd711db83152