These changes fix the basic problem with GIT_DIFF_REVERSE being
broken for text diffs.  The reversed diff entries were getting
added to the git_diff correctly, but some of the metadata was kept
incorrectly in a way that prevented the text diffs from being
generated correctly.  Once I fixed that, it became clear that it
was not possible to merge reversed diffs correctly.  This has a
first pass at fixing that problem.  We probably need more tests
to make sure that is really fixed thoroughly.