Fix for 2020876: num_diff_runes was being calculated incorrectly.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@6588 c06c8d41-db1a-0410-9941-cceddc491573

Created by  haranp  on July 17, 2008
45KSZ2WFRZHB7M2RCJB3PBL777C6GFBAZCKSE3AYDBC6TRUAMWHAC
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