Git: Support remote management

[Tyriar]

#20999

Might be possible already, but it wasn't clear how to add a remote after hitting this:

I also often do the following to sync from upstream, I don't know if this is possible currently:

git checkout https://github.com/Tyriar/xterm.js.git
git remote add upstream https://github.com/sourcelair/xterm.js.git
git fetch upstream
git merge upstream/master
git push

[jpzwarte]

Also missing this feature: i can "Push to..." a new branch to remote "origin" (github), but when i try "git push" from the CLI, it tells me no upstream remote is set and i need to set it. Would be nice if vscode does a "git push -u REMOTE_NAME BRANCH_NAME" when using "Push to...".