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>

Author: peff

git-rebase.sh

@@ -332,11 +332,14 @@ else
 fi
 
 # The tree must be really really clean.
-git update-index --ignore-submodules --refresh || exit
+if ! git update-index --ignore-submodules --refresh; then
+	echo >&2 "cannot rebase: you have unstaged changes"
+	exit 1
+fi
 diff=$(git diff-index --cached --name-status -r --ignore-submodules HEAD --)
 case "$diff" in
-?*)	echo "cannot rebase: your index is not up-to-date"
-	echo "$diff"
+?*)	echo >&2 "cannot rebase: your index contains uncommitted changes"
+	echo >&2 "$diff"
 	exit 1
 	;;
 esac