pgsql: Add script major_release_split to simplify creating release notes

Log Message:
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Add script major_release_split to simplify creating release notes for
multiple releases.

Added Files:
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pgsql/src/tools:
major_release_split (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/tools/major_release_split?rev=1.1&content-type=text/x-cvsweb-markup)