[PATCH] emacs: Use a single buffer invisibility spec to fix quadratic search cost.

[servilio]

Hi,

Given that this change is about display of search results, I have the
suspicion that the following two factors might be more relevant:

- size of the Emacs frame: bigger would mean more threads to show

- composition of the search results, specially length of the threads
displayed, as the longer they go, the more hidden text there will be

And because of this, though you might already have a proper method for
measuring it, the results are different. I thought about this because
my searches have only ~180 threads, yet I could notice the difference.

Servilio