pgsql: Simplify relative-path logic, on the assumption that compiled-in

Log Message:
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Simplify relative-path logic, on the assumption that compiled-in paths
won't have trivial differences in their common prefix.

Modified Files:
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pgsql/src/port:
path.c (r1.43 -> r1.44)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/path.c.diff?r1=1.43&r2=1.44)