pgsql: Fix integer-to-bit-string conversions to handle the first

Log Message:
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Fix integer-to-bit-string conversions to handle the first fractional byte
correctly when the output bit width is wider than the given integer by
something other than a multiple of 8 bits.

This has been wrong since I first wrote that code for 8.0 :-(. Kudos to
Roman Kononov for being the first to notice, though I didn't use his
patch. Per bug #5237.

Tags:
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REL8_3_STABLE

Modified Files:
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pgsql/src/backend/utils/adt:
varbit.c (r1.57 -> r1.57.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varbit.c?r1=1.57&r2=1.57.2.1)