Add git stashing

[Krzysztof-Cieslak]

Addresses #1904 which is one of the oldest and most upvoted issues.

Adds 3 new commands for git integration:

• git stash
• git stash pop using latest stash
• git stash pop with choosing stash

[msftclas]

@Krzysztof-Cieslak,
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[Krzysztof-Cieslak]

CC: @joaomoreno

[joaomoreno]

@Krzysztof-Cieslak We don't really need to have the stashes in the model itself. This will just make git status slower since we'd need to call git once more in order to get the stashes. You can just make this call once one of the stash actions runs. Can you fix that?

[Krzysztof-Cieslak]

Yes, I'll rebase and fix it.

[Krzysztof-Cieslak]

Ready to review again.

[joaomoreno]

@Krzysztof-Cieslak

These changes are not formatted and violate some linting rules we have set. We have configured a git pre-commit hook to validate these for you, which gets set up when you run ./scripts/npm.sh install. You were only able to commit by not running that or by commenting out the pre-commit hook. Next time, please commit while running through these rules. I've fixed those issues for you on this one.

Also, this goes into infinite recursion. Next time, please run your changes before handing them over to us.

Also, this condition will fail when index is zero.

Also, I've added functionality to provide a stash message when stashing changes.

I've fixed all that up.

We do appreciate your contributions very much. But next time, try to follow the guidelines better so we make this smoother.

[Krzysztof-Cieslak]

Sorry for the trouble @joaomoreno, I haven't paid enough attention to this PR, indeed. My bad :(