See a bunch of discussion in IRC. Upshot is that this:
```
try {
list($xml, $stderr) = $repository->execxRemoteCommand(
'log --xml --quiet --limit %d %s',
$limit,
$repository->getSubversionBaseURI($at_rev));
} catch (CommandException $ex) {
$stderr = $ex->getStdErr();
if (preg_match('/(path|File) not found/', $stderr)) {
// We've gone all the way back through history and this path was not
// affected by earlier commits.
break;
}
throw $ex;
}
```
...should fail hard if `$at_rev` is `HEAD`, because this indicates a configuration error.