rename BlobType and add submodule to it

This was badly named, it's a not a blob necessarily, but anything that a
tree can refer to.

Also removed the Show instance which was used for serialization to git
format, instead use fmtTreeItemType.

This commit was supported by the NSF-funded DataLad project.

Git/Repair.hs

@@ -396,10 +396,10 @@ rewriteIndex r
 		void cleanup
 		return $ map fst3 bad
   where
-	reinject (file, Just sha, Just mode) = case toBlobType mode of
+	reinject (file, Just sha, Just mode) = case toTreeItemType mode of
 		Nothing -> return Nothing
-		Just blobtype -> Just <$>
-			UpdateIndex.stageFile sha blobtype file r
+		Just treeitemtype -> Just <$>
+			UpdateIndex.stageFile sha treeitemtype file r
 	reinject _ = return Nothing
 
 newtype GoodCommits = GoodCommits (S.Set Sha)