[RFC] Faster git grep.

Hi, 

When I do git grep then with big codebase (gcc) it executes slowly. I am thinking to add option to speed up search time.

One solution would be to use same trick as was done in google code. Build and keep database of trigraphs and which files contain how many of them. When querry is made then check only these files that have appropriate combination of trigraphs.

Updating database would be relatively inexpensive compared to disk access we need to do anyway.

A space usage might be problem so which is why I decided go option route.

Comments, pointers?

Ondra