Use emacs diff colors

[Azkae]

Improve syntax highlighting by sending the emacs diff colors to delta.
This way, the colors should better match the user's theme/background.

This should also fix #28.

A few notes:

• magit-delta-mode is now a global mode.

• Since we are using diff faces, we need to have the correct faces already loaded when enabling magit-delta-mode. For example, if we enable magit-delta-mode before setting up a theme, we would use the wrong colors.

  Using (add-hook 'magit-mode-hook (lambda () (magit-delta-mode +1))) works since by the time we turn on magit mode, the faces should be correct.

I have also added magit-delta-max-size to avoid running delta on large files. It's easier to do now that magit-delta-mode is a global mode.
(I can move it to later PR if you prefer).

Before:

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After:

[mmahmoudian]

@dandavison Would it be possible to push this PR forward. The current color scheme makes it hard to read the diff in magit, plus the magit-delta-max-size is very handy when dealing with Guix package files (although I wish it was introduced in a separate PR just for the sake of clarity).

Thank you @dandavison for both git-delta and magit-delta, and thank you @Azkae for the PR.

[dandavison]

Hi @Azkae and @mmahmoudian.

@Azkae thanks very much for this work! I have not been doing a good job recently keeping up with PRs and issues, but let me get this running locally and see if I encounter any issues.

@mmahmoudian thanks for the ping. Are you able to run this branch yourself locally to test it out, and to solve the problem for yourself at least?