Introduce a new binary diff callback to provide the actual binary
delta contents to callers.  Create this data from the diff contents
(instead of directly from the ODB) to support binary diffs including
the workdir, not just things coming out of the ODB.

Mark the `old_file` and `new_file` sides of a delta with a new bit,
`GIT_DIFF_FLAG_EXISTS`, that introduces that a particular side of
the delta exists in the diff.

This is useful for indicating whether a working directory item exists
or not, in the presence of a conflict.  Diff users may have previously
used DELETED to determine this information.

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.

When checking out with a case-insensitive working directory, we
want to change the case of items in the working directory to
reflect changes that occured in the checkout target.  Diff now
has an option to break case-changing renames into delete/add.

Conflicts:
	include/git2/diff.h