use headExists

Turns out that 7be690f326 broke the
test suite on the i386ancient builder. There, git show-ref --verify HEAD
fails with "'HEAD' - not a valid ref". Apparently git 2.1.4 didn't
support that.

headExists works there and does the same thing.

Annex/WorkTree.hs

@@ -78,7 +78,7 @@ ifAnnexed file yes no = maybe no yes =<< lookupFile file
  - an annex object. If so, make the unlocked file use that content.
  -}
 scanUnlockedFiles :: Annex ()
-scanUnlockedFiles = whenM (inRepo $ Git.Ref.exists Git.Ref.headRef) $ do
+scanUnlockedFiles = whenM (inRepo Git.Ref.headExists) $ do
 	showSideAction "scanning for unlocked files"
 	Database.Keys.runWriter $
 		liftIO . Database.Keys.SQL.dropAllAssociatedFiles