pgsql: Add pgcvslog '-d' capability to allow stripping of commit

Log Message:
-----------
Add pgcvslog '-d' capability to allow stripping of commit messages that
have back branch activity. This will be useful for creating release
notes for major releases.

Modified Files:
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pgsql/src/tools:
pgcvslog (r1.36 -> r1.37)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/tools/pgcvslog?r1=1.36&r2=1.37)