handle sync's use of setCurrentDirectory to work with relative paths

I think this is the last problimatic setCurrentDirectory. I also audited
for extrnal commands that git-annex might run with cwd = foo, and did not
find any that were passed any FilePath that might be absolute.

Command/Sync.hs

@@ -29,6 +29,7 @@ import Annex.Hook
 import qualified Git.Command
 import qualified Git.LsFiles as LsFiles
 import qualified Git.Branch
+import qualified Git.Types as Git
 import qualified Git.Ref
 import qualified Git
 import qualified Remote.Git
@@ -107,7 +108,7 @@ seek rs = do
  - of the repo. This also means that sync always acts on all files in the
  - repository, not just on a subdirectory. -}
 prepMerge :: Annex ()
-prepMerge = liftIO . setCurrentDirectory =<< fromRepo Git.repoPath
+prepMerge = Annex.changeDirectory =<< fromRepo Git.repoPath
 
 syncBranch :: Git.Ref -> Git.Ref
 syncBranch = Git.Ref.under "refs/heads/synced" . fromDirectBranch