Drastically speed up formatting when ignore or config file exist.

[belav]

closes #594

[shocklateboy92]

Okay, I not sure what the magic bit that caused the massive performance improvement is.
But I don't see any glaring code issues, so LGTM.

[belav]

Okay, I not sure what the magic bit that caused the massive performance improvement is. But I don't see any glaring code issues, so LGTM.

Creating the ignore file is somewhat slow. Before these changes I think it was creating the ignore file once per directory that contained .cs files. Or maybe even once per .cs file that was formatted..
Formatting csharpier-repos with an ignore file took an hour before these changes, vs I think ~7 minutes with these changes or no ignore file.