prevent dropping required content of other file using same content

When two files have the same content, and a required content expression
matches one but not the other, dropping the latter file will fail as it
would also remove the content of the required file.

This will slow down drop (w/o --auto), dropunused, mirror, and move, by one
keys db lookup per file. But I did include an optimisation to avoid a
double db lookup in the drop --auto / sync --content case. I suspect that
dropunused could also use PreferredContentChecked True, but haven't
entirely thought it through and it's rarely used with enough files for the
optimisation to matter.

Sponsored-by: Dartmouth College's Datalad project

Command/Mirror.hs

@@ -69,7 +69,7 @@ startKey o afile (si, key, ai) = case fromToOptions o of
 		( Command.Move.toStart Command.Move.RemoveNever afile key ai si =<< getParsed r
 		, do
 			(numcopies, mincopies) <- getnummincopies
-			Command.Drop.startRemote afile ai si numcopies mincopies key =<< getParsed r
+			Command.Drop.startRemote pcc afile ai si numcopies mincopies key =<< getParsed r
 		)
 	FromRemote r -> checkFailedTransferDirection ai Download $ do
 		haskey <- flip Remote.hasKey key =<< getParsed r
@@ -82,10 +82,11 @@ startKey o afile (si, key, ai) = case fromToOptions o of
 			Right False -> ifM (inAnnex key)
 				( do
 					(numcopies, mincopies) <- getnummincopies
-					Command.Drop.startLocal afile ai si numcopies mincopies key []
+					Command.Drop.startLocal pcc afile ai si numcopies mincopies key []
 				, stop
 				)
   where
 	getnummincopies = case afile of
 		AssociatedFile Nothing -> (,) <$> getNumCopies <*> getMinCopies
 		AssociatedFile (Just af) -> getFileNumMinCopies af
+	pcc = Command.Drop.PreferredContentChecked False