support .git/annex on a different disk than the rest of the repo

The only fully supported thing is to have the main repository on one disk,
and .git/annex on another. Only commands that move data in/out of the annex
will need to copy it across devices.

There is only partial support for putting arbitrary subdirectories of
.git/annex on different devices. For one thing, but this can require more
copies to be done. For example, when .git/annex/tmp is on one device, and
.git/annex/journal on another, every journal write involves a call to
mv(1). Also, there are a few places that make hard links between various
subdirectories of .git/annex with createLink, that are not handled.

In the common case without cross-device, the new moveFile is actually
faster than renameFile, avoiding an unncessary stat to check that a file
(not a directory) is being moved. Of course if a cross-device move is
needed, it is as slow as mv(1) of the data.

Command/Unlock.hs

@@ -46,7 +46,7 @@ perform dest key = do
                 then do
 			liftIO $ do
 				removeFile dest
-				renameFile tmpdest dest
+				moveFile tmpdest dest
 				allowWrite dest
 			next $ return True
                 else error "copy failed!"