filelog: drop index attribute (API)
The previous commit removed the last consumer of the "index" attribute
on the file storage interface. The index is an extremely low-level
data structure that is revlog specific and isn't appropriate to
expose as part of a generic storage API. There may be a market for
an efficient data structure to obtain metadata on every revision for
a file. But if there is, it should be designed using e.g. named
attributes for lookup instead of a list-like of 8-tuples.
Let's drop the attribute from filelog and remove the attribute from
the file storage interface.
Differential Revision: https://phab.mercurial-scm.org/D4720
(binary:application/octet-stream)