Feeding git-annex a long list off directories, eg with xargs can have
2 bad behaviors:

* git-annex runs git ls-files and passes it the same long list. But the git
  ls-files command is longer than the git-annex command often, so it gets
  truncated and some files are not processed.

* It can take a really long time for git-annex to chew through the
  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.
  
  > I think I've managed to speed this up something like
  > 1000x or some such. segmentPaths on an utterly insane list of 6 million
  > files now runs in about 10 seconds. --[[Joey]]