Handle missing "ours" stage in WorkingTreeIterator.hasCrLfInIndex()

Handle missing "ours" stage in WorkingTreeIterator.hasCrLfInIndex()

In a delete-modify conflict with the deletion as "ours" there may be
no stage 2 in the index. Add appropriate null checks. Add a new test
for this case, and verify that the file gets added with a single LF
after conflict resolution with core.autocrlf=true. This matches the
behavior of canonical git for this case.

Bug: 547724
Change-Id: I1bafdb83d9b78bf85294c78325e818e72fae53bc
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>