Use empty temp buffers in mmm-update-mode-info

[dgutov]

Barring some extra special cases, the only reason to use an indirect buffer here I can think of is file-local variables. But we can capture those in the parent buffer (file-local-variables-alist), then let-bind this var in the temp buffer and call hack-local-variables.

This will also bring us a step closer to fixing #25, because sql-mode sets font-lock variables in hack-local-variables-hook.