Auto merge of #5988 - camelid:patch-2, r=ebroto
Auto merge of #5988 - camelid:patch-2, r=ebroto

Syntax-highlight `single_char_push_str` lint

It wasn't being syntax highlighted in the online lint index:

![image](https://user-images.githubusercontent.com/37223377/91666682-8fc02000-eab3-11ea-95fa-6671472712c8.png)

changelog: none

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust