Performance fixes for w_string_path_cat()
Performance fixes for w_string_path_cat()

Summary:
`w_string_path_cat()` shows up as a hotspot when profiling.
`sprintf()` is pretty expensive, so this diff manually concatenates
instead.

Test Plan:
`arc test` -- a few tests are already failing, is this known?
http://pastebin.com/ZbbPA2TC

Reviewers: wez

Differential Revision: https://reviews.facebook.net/D43041

windofthesky/watchman,nodakai/watchman,twitter/watchman,supriyantomaftuh/watchman,exponentjs/watchman,amasad/watchman,yonglehou/watchman,wez/watchman,kevinsawicki/watchman,hanhailong/watchman,dhruvsinghal/watchman,supriyantomaftuh/watchman,yonglehou/watchman,besarthoxhaj/watchman,masoncloud/watchman,montogeek/watchman,yun63/watchman,Ziriax/watchman,windofthesky/watchman,windofthesky/watchman,masoncloud/watchman,facebook/watchman,supriyantomaftuh/watchman,supriyantomaftuh/watchman,dcolascione/laptop-watchman,supriyantomaftuh/watchman,exponentjs/watchman,dcolascione/laptop-watchman,webmasteraxe/watchman,nodakai/watchman,dhruvsinghal/watchman,webmasteraxe/watchman,facebook/watchman,Jonekee/watchman,yun63/watchman,amasad/watchman,kevinsawicki/watchman,twitter/watchman,yangeagle/watchman,facebook/watchman,yonglehou/watchman,kevinsawicki/watchman,sdgdsffdsfff/watchman,yonglehou/watchman,twitter/watchman,bitemyapp/watchman,kwlzn/watchman,nodakai/watchman,yangeagle/watchman,facebook/watchman,supriyantomaftuh/watchman,montogeek/watchman,yun63/watchman,masoncloud/watchman,dcolascione/laptop-watchman,windofthesky/watchman,kwlzn/watchman,amasad/watchman,yangeagle/watchman,amasad/watchman,masoncloud/watchman,michalliu/watchman,yun63/watchman,windofthesky/watchman,kwlzn/watchman,yangeagle/watchman,kevinsawicki/watchman,twitter/watchman,dcolascione/laptop-watchman,wez/watchman,windofthesky/watchman,michalliu/watchman,yangeagle/watchman,amasad/watchman,webmasteraxe/watchman,sdgdsffdsfff/watchman,wez/watchman,masoncloud/watchman,masoncloud/watchman,sdgdsffdsfff/watchman,windofthesky/watchman,besarthoxhaj/watchman,supriyantomaftuh/watchman,besarthoxhaj/watchman,masoncloud/watchman,besarthoxhaj/watchman,twitter/watchman,hanhailong/watchman,Jonekee/watchman,wez/watchman,dcolascione/laptop-watchman,bitemyapp/watchman,webmasteraxe/watchman,wez/watchman,wez/watchman,facebook/watchman,webmasteraxe/watchman,kwlzn/watchman,kevinsawicki/watchman,twitter/watchman,bitemyapp/watchman,yonglehou/watchman,michalliu/watchman,facebook/watchman,michalliu/watchman,amasad/watchman,dhruvsinghal/watchman,Jonekee/watchman,bitemyapp/watchman,yun63/watchman,facebook/watchman,sdgdsffdsfff/watchman,sdgdsffdsfff/watchman,yangeagle/watchman,Jonekee/watchman,kevinsawicki/watchman,webmasteraxe/watchman,sdgdsffdsfff/watchman,wez/watchman,hanhailong/watchman,bitemyapp/watchman,yun63/watchman,yangeagle/watchman,hanhailong/watchman,bitemyapp/watchman,nodakai/watchman,besarthoxhaj/watchman,montogeek/watchman,webmasteraxe/watchman,montogeek/watchman,Jonekee/watchman,michalliu/watchman,dhruvsinghal/watchman,besarthoxhaj/watchman,hanhailong/watchman,wez/watchman,exponentjs/watchman,dhruvsinghal/watchman,dcolascione/laptop-watchman,hanhailong/watchman,yonglehou/watchman,nodakai/watchman,Ziriax/watchman,yonglehou/watchman,dcolascione/laptop-watchman,amasad/watchman,Jonekee/watchman,kevinsawicki/watchman,hanhailong/watchman,webmasteraxe/watchman,yonglehou/watchman,masoncloud/watchman,exponentjs/watchman,sdgdsffdsfff/watchman,Ziriax/watchman,dhruvsinghal/watchman,twitter/watchman,facebook/watchman,nodakai/watchman,dcolascione/laptop-watchman,michalliu/watchman,sdgdsffdsfff/watchman,yun63/watchman,amasad/watchman,hanhailong/watchman,Ziriax/watchman,dhruvsinghal/watchman,Ziriax/watchman,montogeek/watchman,kwlzn/watchman,Ziriax/watchman,exponentjs/watchman,exponentjs/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,supriyantomaftuh/watchman,besarthoxhaj/watchman,kwlzn/watchman,montogeek/watchman,besarthoxhaj/watchman,montogeek/watchman,yun63/watchman,Jonekee/watchman,nodakai/watchman,bitemyapp/watchman,michalliu/watchman,dhruvsinghal/watchman,Jonekee/watchman,montogeek/watchman,facebook/watchman,michalliu/watchman,bitemyapp/watchman,kwlzn/watchman,twitter/watchman