refac(branch checkout): extract checkout logic into a handler

[abhinav]

This seems like further reinforcement of the handler idea.
The commands that need to use checkout functionality
(and verify branch is restacked after checkout)
now get this for free without (&branchCheckoutCmd{...}).Run(...)
and worrying about the N arguments it takes.

[skip changelog]: no user facing changes

[abhinav]

This change is part of the following stack:

• refac(branch track): extract into a Handler #739
  • refac(branch checkout): extract checkout logic into a handler #740 ◀
    • refac(submit): extract into a handler #741

_{Change managed by git-spice.}