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.

Annex/FileMatcher.hs

@@ -70,7 +70,8 @@ parseToken checkpresent checkpreferreddir groupmap t
 			[ ("include", limitInclude)
 			, ("exclude", limitExclude)
 			, ("copies", limitCopies)
-			, ("numcopiesneeded", limitNumCopiesNeeded)
+			, ("lackingcopies", limitLackingCopies False)
+			, ("approxlackingcopies", limitLackingCopies True)
 			, ("inbackend", limitInBackend)
 			, ("largerthan", limitSize (>))
 			, ("smallerthan", limitSize (<))