Add 'Discard All and Pull' button

[navn-r]

This changes introduces a Pull from remote (Discard all) option to the Git Menu

Closes #860

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This may lead into the enhancement of #998, specifically

where all pulls completely overwrote (and pruned) local files

[github-actions]

👈 Launch a binder notebook on branch MLH-Fellowship/jupyterlab-git/feat/discard-all-and-pull

[fcollonval]

Thanks for starting this @navn-r

I left some comments

[fcollonval]

Hey @navn-r I push a commit to fix missing menu items description for JLab >=3.1.

Other than that we should support that new feature as a fallback if the classical pull operation fails.

[fcollonval]

Thanks for pushing this (don't worry about the delay any help at any rate is welcomed).

I left some suggestions and it should be good to go.

Small note: for future contributions, I advice you to use a personal fork rather than the MLH one.

[fcollonval]

Hey @navn-r I tested the latest version, we need to handle better the cases where the user cancel the discard dialog (see the review comments).

Taking over the PR