fromkey: Made idempotent

If the worktree file already exists, and is annexed and uses the same
key, avoid failing, nothing needs to be done.

Had to add lookupFileNotHidden to handle the case where an adjust --hide-missing
is in use, and the worktree file was hidden due to the object content
being missing. lookupFile would return the key of the hidden file,
but it makes sense that after fromkey succeeds, the worktree must
contain the file it was supposed to set up.

Command/FromKey.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2010-2018 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2019 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -12,6 +12,7 @@ module Command.FromKey where
 import Command
 import qualified Annex.Queue
 import Annex.Content
+import Annex.WorkTree
 import qualified Annex
 import qualified Backend.URL
 
@@ -89,9 +90,13 @@ perform key file = do
 	next $ return ok
 
 perform' :: Key -> FilePath -> Annex Bool
-perform' key file = do
-	link <- calcRepo $ gitAnnexLink file key
-	liftIO $ createDirectoryIfMissing True (parentDir file)
-	liftIO $ createSymbolicLink link file
-	Annex.Queue.addCommand "add" [Param "--"] [file]
-	return True
+perform' key file = lookupFileNotHidden file >>= \case
+	Nothing -> do
+		link <- calcRepo $ gitAnnexLink file key
+		liftIO $ createDirectoryIfMissing True (parentDir file)
+		liftIO $ createSymbolicLink link file
+		Annex.Queue.addCommand "add" [Param "--"] [file]
+		return True
+	Just k
+		| k == key -> return True
+		| otherwise -> giveup $ file ++ " already exists with different content"