Incorrect first tab’s size in .md files’s diffs #2970

1. Visit https://github.com/sindresorhus/refined-github/pull/2925/files#diff-cc4aac3e9be04e0413c9520f223b493c
2. The tab is 1-space wide instead of 4

<img width="620" alt="bug" src="https://user-images.githubusercontent.com/1402241/78386588-24deeb80-75de-11ea-9b71-c2220dc759e8.png">

For some reason this doesn't happen to other filetypes, they're displayed with the right size:

<img width="422" alt="no-bug" src="https://user-images.githubusercontent.com/1402241/78386686-4d66e580-75de-11ea-90fd-61a4dfe554cf.png">