git-am --resolved: more usable error message.

After doing the hard work of hand resolving the conflicts in the
working tree, if the user forgets to run update-index to mark
the paths that have been resolved, the command gave an
unfriendly "fatal: git-write-tree: not able to write tree" error
message.  Catch the situation early and give more meaningful
message and suggestion.

Noticed and suggested by Len Brown.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-am.sh

@@ -376,6 +376,13 @@ do
 			echo "No changes - did you forget update-index?"
 			stop_here $this
 		fi
+		unmerged=$(git-ls-files -u)
+		if test -n "$unmerged"
+		then
+			echo "You still have unmerged paths in your index"
+			echo "did you forget update-index?"
+			stop_here $this
+		fi
 		apply_status=0
 		;;
 	esac