svn: only export single path in diff(path!=None)

Subversion can export a particular path at a cost dependent on the size of the file. This speeds up diff() drastically when only looking at a single, smaller file.

```
$> time python testdiff.py /local/pkilgo/puppet-main/ 1 320 trunk/manifests/site.pp > /dev/null

real	0m3.124s
user	0m2.744s
sys	0m0.328s

--- vs --

$> time python testdiff.py /local/pkilgo/puppet-main/ 1 320 trunk/manifests/site.pp > /dev/null

real	0m0.095s
user	0m0.044s
sys	0m0.028s
```