More git_diff_find_similar improvements

More git_diff_find_similar improvements

- Add new GIT_DIFF_FIND_EXACT_MATCH_ONLY flag to do similarity
  matching without using the similarity metric (i.e. only compare
  the SHA).
- Clean up the similarity measurement code to more rigorously
  distinguish between files that are not similar and files that
  are not comparable (previously, a 0 could either mean that the
  files could not be compared or that they were totally different)
- When splitting a MODIFIED file into a DELETE/ADD pair, actually
  make a DELETED/UNTRACKED pair if the right side of the diff is
  from the working directory.  This prevents an odd mix of ADDED
  and UNTRACKED files on workdir diffs.