Significantly sped up processing of large numbers of directories passed to a single git-annex command. (try 2)

New approach is to do it the expensive way for the first 100 paths
on the command line, but then assume the user doesn't care about order too
much and fall back to the cheap way that does not preserve order.

doc/bugs/feeding_git_annex_with_xargs_can_fail.mdwn

@@ -11,3 +11,5 @@ Feeding git-annex a long list off directories, eg with xargs can have
   git-ls-files results. There is probably an exponential blowup in the time
   relative to the number of parameters. Some of the stuff being done to
   preserve original ordering etc is likely at fault.
+
+  > [[fixed|done]] --[[Joey]]