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/Uninit.hs

@@ -105,7 +105,7 @@ removeUnannexed = go []
 	go c [] = return c
 	go c (k:ks) = ifM (inAnnexCheck k $ liftIO . enoughlinks)
 		( do
-			lockContent k removeAnnex
+			lockContentExclusive k removeAnnex
 			go c ks
 		, go (k:c) ks
 		)