sync --content: New option that makes the content of annexed files be transferred.

Similar to the assistant, this honors any configured preferred content
expressions.

I am not entirely happpy with the implementation. It would be nicer if
the seek function returned a list of actions which included the individual
file gets and copies and drops, rather than the current list of calls to
syncContent. This would allow getting rid of the somewhat reundant display
of "sync file [ok|failed]" after the get/put display.

But, do that, withFilesInGit would need to somehow be able to construct
such a mixed action list. And it would be less efficient than the current
implementation, which is able to reuse several values between eg get and
drop.

Note that currently this does not try to satisfy numcopies when
getting/putting files (numcopies are of course checked when dropping
files!) This makes it like the assistant, and unlike get --auto
and copy --auto, which do duplicate files when numcopies is not yet
satisfied. I don't know if this is the right decision; it only seemed to
make sense to have this parallel the assistant as far as possible to start
with, since I know the assistant works.

This commit was sponsored by Øyvind Andersen Holm.

Command/Get.hs

@@ -59,7 +59,11 @@ perform key afile = stopUnless (getViaTmp key $ getKeyFile key afile) $
 {- Try to find a copy of the file in one of the remotes,
  - and copy it to here. -}
 getKeyFile :: Key -> AssociatedFile -> FilePath -> Annex Bool
-getKeyFile key afile dest = dispatch =<< Remote.keyPossibilities key
+getKeyFile key afile dest = getKeyFile' key afile dest
+	=<< Remote.keyPossibilities key
+
+getKeyFile' :: Key -> AssociatedFile -> FilePath -> [Remote] -> Annex Bool
+getKeyFile' key afile dest = dispatch
   where
 	dispatch [] = do
 		showNote "not available"