Support manually created branches

[adjerbetian]

Use case:

• I start a stack my-stack
• I create commits 5 c1, c2, ...
• Then I realize I could have split commits in 2 stacked branches: 2 commits + 3 commits
• I create a branch pre-work from the 2 commit
• The history looks like this:

c0    main
c1
c2    pre-work
c3
c4
c5    my-stack

• When I do gh tack view, the branch pre-work is not recognized, and I don't know how to make it recognized

[skarim]

This can now be done via the gh stack add command (which accepts existing branches) or the gh stack modify command (which allows modifications of a stack). For more complex operations like splitting commits into new branches, let's use #263.