indent with tabs not spaces

Found these with:
git grep "^  " $(find -type  f -name \*.hs) |grep -v ':  where'

Unfortunately there is some inline hamlet that cannot use tabs for
indentation.

Also, Assistant/WebApp/Bootstrap3.hs is a copy of a module and so I'm
leaving it as-is.

Git/DiffTree.hs

@@ -53,7 +53,7 @@ diffIndex ref = diffIndex' ref [Param "--cached"]
 diffWorkTree :: Ref -> Repo -> IO ([DiffTreeItem], IO Bool)
 diffWorkTree ref repo =
 	ifM (Git.Ref.headExists repo)
-                ( diffIndex' ref [] repo
+		( diffIndex' ref [] repo
 		, return ([], return True)
 		)