Add setting to make Git: Fetch (Prune) default behaviour for Git Fetch And Pull

[tgoncuoglu]

There have been some closed issues such as #78366 that are somewhat similar to this, but with a key difference: they assume local branch is not deleted. However, the problem is: when a remote branch is deleted and its local branch is deleted too, VSCode still does show not-existing-anywhere branches in branch checkout pull-down until Git: Fetch (Prune) is used. I have confirmed that those branches in checkout pulldown still persist after restarting vscode.

While this may indeed be about git itself, since Git: Fetch (Prune) has been implemented in vscode; my suggestion is that it is only natural to add an option in settings such as git.pruneOnFetch: True to append the --prune flag to git fetch and pull operations, making this default behaviour if user so chooses. This will avoid the need to run Git: Fetch (Prune) once in a while, and will have the benefit of keeping the checkout pulldown clean.

[joaomoreno]

Let's add a setting, but not have it enabled by default.

[Manish-Giri]

Hello, can I work on this?