Simplify _mon_on_interesting_grid() by reusing _is_native_in_branch()

where possible.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@5314 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on May 28, 2008
L5H6B2WM4OOVGVCZOT3Z7DC2MOUVDLZ4DWRA6L6J3EUXF55USTZQC
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