pgsql: Fix coverage targets so that HTML view is reliably updated when

Log Message:
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Fix coverage targets so that HTML view is reliably updated when test data
changes. Add some documenting comments.

Modified Files:
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pgsql/src:
Makefile.global.in (r1.243 -> r1.244)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/Makefile.global.in?r1=1.243&r2=1.244)