Improve performance by merging View.substr calls

[jfcherng]

The original implementation may call View.substr "the length of the bracket content region" times. Now it's just called once.

In my daily setup which uses

    "search_threshold": 20000, // 5000 by default
    "content_highlight_bar": true,
    "align_content_highlight_bar": true,

This may reduce numbers of View.substr calls from 10k to just 1.

[facelessuser]

Looks good. I'll have to verify functionality, but I'm not expecting any issues.

[facelessuser]

@gir-bot lgtm