move InodeSentinal from direct mode code to its own module

Will be used outside of direct mode for v6 unlocked files, and is already
used outside of direct mode when adding files to annex.

Annex/Direct.hs

@@ -1,4 +1,7 @@
 {- git-annex direct mode
+ -
+ - This is deprecated, and will be removed when direct mode gets removed
+ - from git-annex.
  -
  - Copyright 2012-2014 Joey Hess <id@joeyh.name>
  -
@@ -36,6 +39,7 @@ import Annex.VariantFile
 import Git.Index
 import Annex.Index
 import Annex.LockFile
+import Annex.InodeSentinal
 
 {- Uses git ls-files to find files that need to be committed, and stages
  - them into the index. Returns True if some changes were staged. -}