fix: performance improvements when meta_generator is disabled

[SukkaW]

What does it do?

When hexo.config.meta_generator is false, return before cheerio.load.

Discussion: #3663 (comment)

How to test

Environment

• Travis CI
• theme-suka/hexo-theme-unit-test as basic hexo site
• hexo-theme-suka
• hexo-many-posts: 300 posts that have 1 category, 5 tags, all markdown styles and large amounts of code blocks
• All the css, js & images are deleted before hexo clean && hexo g to make sure every file generated is rendered HTML.

Screenshots

Raw build log can be found here.

Test A

• The latest master branch of Hexo from git+https://github.com/hexojs/hexo.git
• meta_generator set to true

694 Files / 20 sec

Test B

• The latest master branch of Hexo from git+https://github.com/hexojs/hexo.git
• meta_generator set to false

694 Files / 20 sec

Test C

• The latest master branch of Hexo from git+https://github.com/hexojs/hexo.git
• meta_generator set to false
• sed -i "s|filter.register('after_render:html', require('./meta_generator'))|//filter.register('after_render:html', require('./meta_generator'))|g" ./node_modules/hexo/lib/plugins/filter/index.js to comment out https://github.com/hexojs/hexo/blob/master/lib/plugins/filter/index.js#L14

694 Files / 8.62 sec

Test D

• The latest master branch of Hexo from git+https://github.com/sukkaw/hexo.git#meta-generator-config
• meta_generator set to true

694 Files / 20 sec

Test E

• The latest master branch of Hexo from git+https://github.com/sukkaw/hexo.git#meta-generator-config
• meta_generator set to false

694 Files / 8.56 sec

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage increased (+0.0008%) to 97.154% when pulling 66fa0e7 on SukkaW:meta-generator-config into 48b506d on hexojs:master.

[curbengh]

This is the proper syntax to use.