Make diff file view easier to use on mobile screens

Viewing diffs on a mobile screen is a bit of an awkward experience at the moment. Here are a few issues (by no means complete):

Before

Tabs are scrunched

Filenames take too much room, buttons cluttered

After

This MR makes a few tweaks to make this a bit better. It just addresses a few issues, but there is plenty of room for improvement (e.g. shrink fonts, etc.):

Eliminate padding to make tabs fit

Make filenames, buttons more readable

This screenshot allows the filename to use the whole row, omits the file mode changes, and puts the buttons centered in the view:

Towards a better mobile experience: #2787 (closed)