Changed merged region tracking to use object pointers rather than

Changed merged region tracking to use object pointers rather than
index pointers so we don't have to make sure the indices are up to date.
PR: 22922
Submitted by:	xuemin@appresso.com (Xuemin Guan)
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git-svn-id: https://svn.apache.org/repos/asf/jakarta/poi/branches/REL_2_BRANCH@353343 13f79535-47bb-0310-9956-ffa450edef68