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

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        varbit.c (r1.58 -> r1.59)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varbit.c?r1=1.58&r2=1.59)