Add a validation script

Add a validation script

Add a validation script. Using an existing dump known to be correct
(possibly generated using `svnsync` and `svnadmin dump --deltas`), it
compares the outputs produced by `svnadmin load` when fed with this
dump and the dump from the program.

The arguments added are:
    --svnadmin-dump=<file>      existing "svnadmin dump"
    --svndumpr-dump=<file>      existing "svndumpr" dump
    --repos=<url-or-path>       SVN repos URL, or local repos
    -r<revision>                end revision to dump/cut
    --ignore-existing           regenerate svnadmin-dump even if
                                it already exists
    --make                      run "make svndumpr" prior to svnrdump
    generate                    as with the old parameter, but now
                                implied by default
    validate                    as with the old parameter, but now
                                implied by default

The end result is that the only step technically needed to test
validation is:

./validate.sh --make --repos=/path/to/local/repos

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Will Palmer <wpalmer@gmail.com>