git: Add support for git pull with rebase

[Sathiyaraman-M]

• Adds a new action git::PullRebase which adds --rebase in the final command invoked by existing Git-Pull implementation.
• Includes the new action in "Fetch/Push" button in the Git Panel (screenshot below)
• Adds key-binding for git::PullRebase in all three platforms, following the existing key-binding patterns (ctrl-g shift-down)
• Update git docs to include the new action.

Sidenote: This is my first ever OSS contribution

Screenshot:

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Release Notes:

• Git: Added git: pull rebase for running git pull --rebase.

[cla-bot]

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[Sathiyaraman-M]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[cole-miller]

Thanks, one piece of feedback but seems good otherwise!

[cole-miller]

I think we shouldn't pass --no-rebase if rebase is false here, so you get the configured behavior for the branch (which might be either merge or rebase)--this is how git::Pull works in Zed now and we should maintain that behavior.