benchmarked numcopies .gitattributes in preferred content

Checking .gitattributes adds a full minute to a git annex find looking for
files that don't have enough copies. 2:25 increasts to 3:27. I feel this is
too much of a slowdown to justify making it the default. So, exposed two
versions of the preferred content expression, a slow one and a fast but
approximate one.

I'm using the approximate one in the default preferred content expressions
to avoid slowing down the assistant.

Types/StandardGroups.hs

@@ -93,6 +93,8 @@ notArchived :: String
 notArchived = "not (copies=archive:1 or copies=smallarchive:1)"
   	
 {- Most repositories want any content that is only on untrusted
- - or dead repositories, or that otherwise does not have enough copies. -}
+ - or dead repositories, or that otherwise does not have enough copies.
+ - Does not look at .gitattributes since that is quite a lot slower.
+ -}
 lastResort :: String -> PreferredContentExpression
-lastResort s = "(" ++ s ++ ") or numcopiesneeded=1"
+lastResort s = "(" ++ s ++ ") or approxlackingcopies=1"