This updates the "git_cmp_c" and "git_cmp_rs" links in the
`gix_diff::tree::function::diff` documentation comment.

- The significant change is to the "git_cmp_rs" link (the second
  link), which accounts for how files have renamed and how code has
  been moved, and also changed in GitoxideLabs#849 to fix issue GitoxideLabs#848.

  This hyperlink change completes the third task listed in GitoxideLabs#1627.

- The other change is to update the "git_cmp_c" link at the same
  time. That code has not changed, and this is just referring to a
  later commit (the current tip of the master branch in git/git).

  The reason to also do this change is to make it easier to verify,
  both now and for anyone reading the documentation, that that link
  remains current.