rebase: improve error messages about dirty state

If you have unstaged changes in your working tree and try to
rebase, you will get the cryptic "foo: needs update"
message, but nothing else.  If you have staged changes, you
get "your index is not up-to-date".

Let's improve this situation in two ways:

 - for unstaged changes, let's also tell them we are
   canceling the rebase, and why (in addition to the "needs
   update" lines)

 - for the staged changes case, let's use language that is a
   little more clear to the user: their index contains
   uncommitted changes

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

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