Check if site is in incremental mode optimally

[ashmaroli]

• This is a 🔨 code refactoring change.

Summary

Jekyll::Site#incremental? takes an optional argument which is an empty Hash by default:

jekyll/lib/jekyll/site.rb

Lines 367 to 369 in f8c7208

	def incremental?(override = {})
	override["incremental"] || config["incremental"]
	end

Since the empty hash is a default parameter, every call to the method allocates an empty Hash.
Therefore, calling the method while rendering an include file can allocate numerous Hashes unnecessarily.

The solution is then to access the config value directly.

[ashmaroli]

Thank you @DirtyF :)
@jekyllbot: merge +fix