When diffing against an index, return a new `GIT_DELTA_CONFLICTED`
delta type for items that are conflicted.  For a single file path,
only one delta will be produced (despite the fact that there are
multiple entries in the index).

Index iterators now have the (optional) ability to return conflicts
in the index.  Prior to this change, they would be omitted, and callers
(like diff) would omit conflicted index entries entirely.