Test to ensure that when status updates an index, it does not alter
the original mode for file types that are not supported (eg, symlinks
on Windows).

These tests want to test that we don't recalculate entries which match
the index already. This is however something we force when truncating
racily-clean entries.

Tick the index forward as we know that we don't perform the
modifications which the racily-clean code is trying to avoid.

Since a diff entry only concerns a single entry, zero the information
for the index side of a conflict.  (The index entry would otherwise
erroneously include the lowest-stage index entry - generally the
ancestor of a conflict.)

Test that during status, the index side of the conflict is empty.

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 adding a conflict for some path, remove the staged entry.
Otherwise, an illegal index (with both stage 0 and high-stage
entries) would result.

When we discover that we want to keep a negative rule, make sure to
clear the error variable, as it we otherwise return whatever was left by
the previous loop iteration.