pgbuildfarm - client-code: Use git clean to remove build products left

Log Message:
-----------
Use git clean to remove build products left by vpath builds, following
a suggestion from Robert Haas. This removes the need to do any ignore
file processing at all with git. We still don't use git clean at the
checkout stage - if the user has left stuff lying around it might be
important to them, so instead of blowing it away we just complain loudly
and refuse to proceed until they clean it up.

This cleans up the SCM API a bit. find_ignore() is removed, and cleanup()
is added. The CVS subclass retains a private find_ignore() method. The second
argument to checkout() is also now removed.

Modified Files:
--------------
    client-code:
        run_build.pl (r1.108 -> r1.109)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbuildfarm/client-code/run_build.pl?r1=1.108&r2=1.109)
    client-code/PGBuild:
        SCM.pm (r1.2 -> r1.3)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbuildfarm/client-code/PGBuild/SCM.pm?r1=1.2&r2=1.3)