Minor changes for rewrites

Commit `f676ed5` is to ensure that we always split rewrites.  Currently, if the only change is a rewrite, it does not get split in `git_diff_find_similar`.  (However it would be split if another rewrite - one impacting a rename - were to exist.)

Commit `d7c3a10` changes the behavior of `git_status` to more closely emulate `git-status(1)`, where rewrites will contribute to renames, but otherwise will not be shown.  That is to say if you were to completely rewrite file `c`, while renaming `a` to `b` and `b` to `a`, you expect the status:

    #       renamed:    b -> a
    #       renamed:    a -> b
    #       modified:   c

(Currently we would display `c` as an add/delete pair.)