i#84172 RTF import: handle nested groups inside redlines

The problem was that in case we had {\revised foo {bar}}, then we only
started a redline before "foo", but we ended it twice: during handling
of both "}". Make sure that the end of the redline is only handled by
the second one.

Change-Id: I41ab8b486e8615d3479fa0fadd6016476859b4b2