convert srcref to a sha

This fixes pushing a new ref that is the same as something already
pushed. In findotherprereq, it compares two shas, which didn't work when
one is actually not a sha but a ref.

This is one of those cases where Sha being an alias for Ref makes it
hard to catch mistakes. One of these days those need to be
differentiated at the type level, but not today..

CmdLine/GitRemoteAnnex.hs

@@ -238,14 +238,16 @@ push st rmt ls = do
 			, trackingrefs
 			)
 		in calc rs =<< case srcRef r of
-			Just srcref
-				| forcedPush r -> okresp $
-					M.insert tr srcref trackingrefs
-				| otherwise -> ifM (isfastforward srcref tr)
-					( okresp $
-						M.insert tr srcref trackingrefs
-					, errresp "non-fast-forward"
-					)
+			Just srcref -> inRepo (Git.Ref.sha srcref) >>= \case
+				Just sha
+					| forcedPush r -> okresp $
+						M.insert tr sha trackingrefs
+					| otherwise -> ifM (isfastforward sha tr)
+						( okresp $
+							M.insert tr sha trackingrefs
+						, errresp "non-fast-forward"
+						)
+				Nothing -> errresp "unknown ref"
 			Nothing -> okresp $ M.delete tr trackingrefs
 	
 	-- Check if the push is a fast-forward that will not overwrite work