refresh_index: make porcelain output more specific

refresh_index: make porcelain output more specific

If you have a deleted file and a porcelain refreshes the
cache, we print:

  Unstaged changes after reset:
  M	file

This is technically correct, in that the file is modified,
but it's friendlier to the user if we further differentiate
the case of a deleted file (especially because this output
looks a lot like "diff --name-status", which would also make
the distinction).

Similarly, we can distinguish typechanges ("T") and
intent-to-add files ("A"), both of which appear as just "M"
in the current output.

The plumbing output for all cases remains "needs update" for
historical compatibility.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>