add lockContentShared

Also, rename lockContent to lockContentExclusive

inAnnexSafe should perhaps be eliminated, and instead use
`lockContentShared inAnnex`. However, I'm waiting on that, as there are
only 2 call sites for inAnnexSafe and it's fiddly.

Command/Move.hs

@@ -123,7 +123,7 @@ toPerform dest move key afile fastcheck isthere =
 			finish
   where
 	finish
-		| move = lockContent key $ \contentlock -> do
+		| move = lockContentExclusive key $ \contentlock -> do
 			removeAnnex contentlock
 			next $ Command.Drop.cleanupLocal key
 		| otherwise = next $ return True