pgsql: Update CVS documentation to be more current and add documentation

Log Message:
-----------
Update CVS documentation to be more current and add documentation about
git mirror.

Remove information about cvsup and documentation that's more about cvs
than our use of cvs.

Backpatch to 8.4 so we get the git information up on the website as
soon as possible.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        cvs.sgml (r1.51 -> r1.52)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/cvs.sgml?r1=1.51&r2=1.52)
        installation.sgml (r1.328 -> r1.329)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/installation.sgml?r1=1.328&r2=1.329)