Add diff options as icon buttons

[stdout-se]

Overview

Adds toolbar controls to the right side of the diff file header for common diff display options, with matching entries in Preferences -> Code.

Toolbar

• Word wrap (toggle) — existing preference; new shortcut in the header (still available from the diff context menu and Preferences).
• Formatting marks (toggle) — show tab and space characters in the diff text.
• Comparison method (menu) — choose how Git compares lines when showing a patch in the diff view.

Preferences

New settings under Code:

• Show formatting marks (tabs and spaces)
• Comparison method (radio list, same four modes as the header menu)

(Note: Word wrap was already in Preferences, no change to this)

Comparison method

Controls how git diff is run when loading a patch for the diff view. Options map to Git’s built-in flags:

Option	Git flags
Recognize line endings and white space differences (this is the default)	None
Ignore line ending differences	--ignore-cr-at-eol
Ignore line ending and white space length differences	--ignore-cr-at-eol, --ignore-space-change
Ignore line ending differences and all white space differences	--ignore-cr-at-eol, --ignore-all-space

Note: Comparison method affects how patches are displayed on-screen only. This does not affect exporting patches, reverting files, etc. This still uses the precise methods.

Changing the comparison method reloads the current patch (re-runs git diff). Word wrap and formatting marks apply immediately without reloading.

Icons

Notes about icons I have used: All icons are MIT Licensed according to these pages:
Update: See comments below.

https://www.svgrepo.com/svg/361077/clear-all
https://www.svgrepo.com/svg/361075/circle-slash
https://www.svgrepo.com/svg/361072/circle-large-filled
https://www.svgrepo.com/svg/361073/circle-large-outline
https://www.svgrepo.com/svg/361403/whitespace
https://www.svgrepo.com/svg/361405/word-wrap

[stdout-se]

Reference link: https://git-scm.com/docs/diff-options#Documentation/diff-options.txt-ignore-space-at-eol

[stdout-se]

License update: I found the proper reference repo for the icons:
Reference links:
https://github.com/microsoft/vscode-codicons
https://microsoft.github.io/vscode-codicons/dist/codicon.html

License:
https://github.com/microsoft/vscode-codicons/blob/main/LICENSE

While the license for the code in the repo is MIT, the icons are under The license is Creative Commons Attribution 4.0 International. Not great. Not terrible, I guess.

[jorio]

Super neat!! I've wanted to do this for a long time, this'll be sure to make people happy. It looks well polished from what I can see. Awesome!

I'll cook up some SVG icons by hand so we don't have to worry about licensing.

Please, can you convert your identifiers to camelCase for consistency with the rest of the code? (The only place with snake_case is porcelain.py because it's an extension to pygit2; the rest of the code more or less follows Qt's naming conventions.)

I will review the rest of your code in detail soon. Thank you for your hard work!

[stdout-se]

Fixed name case, also added a simple test for the formatting marks button