style: chained assignment

[curbengh]

What does it do?

Avoid issue #3985, to avoid calling a function twice (e.g. dirname()). The returned result of a function is referred instead.

How to test

git clone -b chain-assignment https://github.com/curbengh/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

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

[coveralls]

Coverage remained the same at 97.117% when pulling 3808441 on curbengh:chain-assignment into 5179a6a on hexojs:master.