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.

doc/todo/preferred_content_numcopies_check.mdwn

@@ -59,7 +59,9 @@ Conclusion:
   to instead end with "or numcopiesneeded=1" **done**
 * See if "numcopiesneeded=N" can check .gitattributes without getting
   a lot slower. If now, perhaps add a "numcopiesneededaccurate=N" that
-  checks it.
+  checks it. **done**
+
+[[done]] 
 
 ## Stability analysis