allow to add existing branch to stack

[da1z]

i created stack from main to my current branch. but between them i have few branches and i can not find a way to add those branches to stack

[da1z]

ok was able to do it with gh stack init --adopt branch-2 branch-1 branch-3 but would be nice to be able to use gh stack add 'existing branch'

[singalen]

But this only adds existing branches to a new stack.

I have a use case to add existing branch to existing stack. I believe, we need gh stack add --adopt

[henzeb]

I think we can do that with gh stack link. This cli tool is, in comparison with graphite, not very user friendly.

[skarim]

This has been implemented in #101

gh stack add now supports pulling in existing branches to a stack