Fix group meta query SQL manipulation.

The wildcard operators were acting too greedily, preventing some JOIN clauses
from being recognized and resulting in invalid SQL syntax. This problem had
previously been masked by ideosyncracies of WP's WP_Meta_Query SQL syntax,
which changed in [WP29887].

This change should be considered a band-aid fix for what is already a band-aid
fix. The proper resolution for this issue is to migrate away from the comma
syntax for JOINs used in BP_Groups_Group. See #5874, #5451.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9082 cdf35c40-ae34-48e0-9cc9-0c9da1808c22