Add support for submitting partial stacks

Dear @ezyang,

one of the beautiful things of ghstack is that I don't need to handle so many branches for a stacked PR workflow anymore.
However, I usually have part of the stack I'm working on ready for review and another part (the tail) that is still WIP, where I don't want to open PRs yet, because I don't want to commit (no pun intended) to a final look of my chain of commits that are yet to come and delete them without poluting github with many PRs.

As far as I know ghstack submits/updates from the HEAD up.
I now see that submitting a partial stack is as easy as checking out the respective most-downstream commit and running ghstack from this detached HEAD state. This is however not perfect because next I need to rebase my working branch on top of the detached HEAD (which has a new commit ID now!) etc.

I came up with a porcelain bash function ghpartial <commit sha> that takes care of what would ideally happen if ghstack <commit sha> would be run (in my humble opinion :) )

ghpartial () {
	BRANCH=$(git rev-parse --abbrev-ref HEAD)
	SUBMIT_BRANCH=$BRANCH-submit
	SUBMIT_COMMIT=$1
	DIRTY=false
	if [[ ! -z $(git status -s) ]]
	then
		echo "stashing uncommited changes..."
		DIRTY=true
		git stash -u
	fi
	if ! git show-ref --quiet refs/heads/$SUBMIT_BRANCH
	then
		echo "creating submit branch"
		git checkout -b $SUBMIT_BRANCH
	fi
	git checkout $SUBMIT_BRANCH
	git reset --hard $SUBMIT_COMMIT
	ghstack
	git checkout $BRANCH
	git rebase $SUBMIT_BRANCH
	if [ "$DIRTY" = true ]
	then
		git stash pop
	fi
	echo "All done. continue chipping away..."
}

(This keeps a branch around that shows how much of the work was submitted already, which is nice for my personal needs because I rebase the current working branch a lot and then it can be handy to have this marker in a prettyfied git log --graph --full-history view)

Would you like a PR for adding such an argument to ghstack natively? Maybe you have ideas how you would like it to be implemented?

Anyway, this tool is just a blaze to use and I convinced half of the people on my team to switch over after presenting it to them recently! Incredibly useful work ❤️

Best,
Zacharias

Start at the ghstack CLI entry point and trace how submission currently proceeds from HEAD. Define the commit-argument behavior around the partial-stack workflow described here, including returning to and rebasing the working branch; done means the selected prefix is submitted without requiring a detached HEAD.