[videoinfoscanner] re-use fasthash if possible

No need to compute a slow hash in case the fast one already succeeded. Same already happens for shows.

@jmarshallnz for review please.
Hmm, not sure you know this.  e.g. what if I add a folder to a folder that previously didn't have one?  Then it previously had a fasthash, but now it won't.

Basically, you have to get the full folder with info here.  You don't need to compute GetPathHash() if you don't want to, though it doesn't cost much to do so.  You could do something like:
```
if (!CanFastHash(items, regexps))
{ // need to use a slow hash
  GetPathHash(items, hash);
  if (dbHash == hash)
    bSkip = true;
}
if (!bSkip)
{ // need to retrieve info etc.
}
```