pgsql: Removes duplicate slashes from the path in canonicalize_path().

Log Message:
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Removes duplicate slashes from the path in canonicalize_path(). It
preserve double leading slashes on Win32.

e.g. ////a////b => /a/b

Modified Files:
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pgsql/src/port:
path.c (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/path.c.diff?r1=1.44&r2=1.45)