Add 'open file' command to context menu on diff tab

[BoscoCHW]

Fixes #941

[github-actions]

👈 Launch a binder notebook on branch BoscoCHW/jupyterlab-git/feature-941

[BoscoCHW]

@fcollonval

I successfully implemented opening a diff-ed file from the context menu. However, since the tab element of the diff window does not have a custom class, I cannot select only the diff tabs to add the command. I am unable to add a class to the diff tab also. Please let me know what I can do. Thank you!

[fcollonval]

@BoscoCHW I looked for a solution, you can add a class to the diff widget title that will allow to set a more specific context menu selector.

You can add a class using diffWidget.title.className = '...'; at

jupyterlab-git/src/commandsAndMenu.tsx

Line 521 in 708d19f

diffWidget.title.closable = true;

[BoscoCHW]

@fcollonval
Hi, I successfully implemented a more specific context menu selector. The 'Open File' command only show up on diff tabs. Please let me know if there are any other changes to make on this PR.

[fcollonval]

Thanks @BoscoCHW

Two minor suggestions and we are good to go.

[fcollonval]

@BoscoCHW I took the liberty to improve a bit your PR to open the file even if it is not in the status file list (aka when the file is not being modified). And I extended the diff widget API to get access to the diff model. That allows a more reliable access to the filename than reading the title attribute of the HTML node.