optimise case of remote that retrieves FileContent, when chunks and encryption are not being used

No need to read whole FileContent only to write it back out to a file in
this case. Can just rename! Yay.

Also indidentially, fixed an attempt to open a file for write that was
already opened for write, which caused a crash and deadlock.

Types/StoreRetrieve.hs

@@ -23,6 +23,10 @@ data ContentSource
 	= FileContent FilePath
 	| ByteContent L.ByteString
 
+isByteContent :: ContentSource -> Bool
+isByteContent (ByteContent _) = True
+isByteContent (FileContent _) = False
+
 -- Action that stores a Key's content on a remote.
 -- Can throw exceptions.
 type Storer = Key -> ContentSource -> MeterUpdate -> Annex Bool