[Enhancement] Additions to the palette

[vanyle]

The command palette #1473 was a great addition, but it lacks some of the most common tasks making it not very useful in its current state.

I believe the most common commands that one might use the command palette for are:

• Checking out a branch by name
• Creating a new branch
• Pushing
• Pulling

Adding these commands would go a long way toward making SourceGit more keyboard friendly.

[love-linger]

I've pushed a new commit that adds Checkout Branch command in repository's command palette.

• For the buttons on the toolbar, I would like to set shortcut keys rather than adding them to the Command Palette.
• For the Push operation, there are many options, and each is relatively important.
• For the Pull operation, since it can only be used on the current branch, it is recommended to directly use the Ctrl+Shift+Down shortcut key.

[AlanIWBFT]

I would like to add another thing here: press Enter to select and apply the first command, which is how the palette works in Fork.

[vanyle]

I'd be happy to contribute pull as a command to the palette.
I believe that the palette is a great way to make shortcuts more discoverable: the first time an action is taken, it can be done with the palette, and there, the user sees the keybinding associated with the action to do it faster when needed later