Update diff2html to version 3

[campersau]

just to keep up with the API changes.

See rtfpessoa/diff2html#297 (comment)

[Hirse]

Are there no changes required to the CSS?

[campersau]

Everything above this line is copied from https://github.com/rtfpessoa/diff2html/blob/4e28c5c3275f6f703a6923d23d001bfb676331a6/src/ui/css/diff2html.css
Everything below this line is customized for ungit.
Maybe we should split these into separate files.

[Hirse]

Maybe add the version number of diff2html here.

[campersau]

Not sure, this part is copied 1:1

[campersau]

Opted to include the css file from npm package so this file now only contains the adjustments.

[campersau]

NEW | OLD

[Hirse]

Thanks for the comparison images.
Where does the difference in height come from?
Also, in the bottom case (red) the line number color changes.

[campersau]

The height difference came from the different font (monospace) and font-size (13px) which I have changed back to the previous values here 14a6e9b

Some other things which changed:

• The lines which contain changes are no longer indented a little bit more than the lines without changes
• The line numbers are now always aligned to the right
• The line numbers are now the same color in line by line vs side by side diffs
• Also fixed the checkbox alignment for patches

Updated screenshots from the new version:

[Hirse]

These don't seem to be necessary any more:
https://developer.mozilla.org/en-US/docs/Web/CSS/user-select#Browser_compatibility

Also, is this not done by diff2html already?

[campersau]

Only in the diff2html UI which we are not using. Also it seems a bit buggy. I have opened rtfpessoa/diff2html#303 for that.