pgbuildfarm - client-code: Provide a new base_port config parameter,

Log Message:
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Provide a new base_port config parameter, that does away with the need
to specify a port for each branch. That means the config file doesn't
need to change with each release any more.

Abstract the SCM operations into an SCM class. Implement two subclasses,
one for CVS and one for Git. Supporting a new SCM if we ever wanted to would
now just be a matter of implementing a new subclass and configuring it in
the SCM factory method.

Modified Files:
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    client-code:
        build-farm.conf (r1.28 -> r1.29)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbuildfarm/client-code/build-farm.conf?r1=1.28&r2=1.29)
        run_build.pl (r1.106 -> r1.107)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbuildfarm/client-code/run_build.pl?r1=1.106&r2=1.107)

Added Files:
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    client-code/PGBuild:
        SCM.pm (r1.1)

(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbuildfarm/client-code/PGBuild/SCM.pm?rev=1.1&content-type=text/x-cvsweb-markup)