feat: improve av stack reorder UX

[twavv]

This was originally going to be just fixing an issue that occurs where running av stack reorder on a stack that has a branch with no commits.

That ended up being comparatively easy to fix (it's essentially just the two line change in git.go), but realized that we didn't actually include any instructions when running av stack reorder, so I added that.

Now, when editing a reorder plan, it looks like:

stack-branch 2023-10-20-blegg-1 --trunk main@49331c9878615a5227fe56ede8521f7e5d8462e7  # this branch has no commits

# Instructions:
#
# Commands:
# sb, stack-branch <branch-name> [--parent <parent-branch-name> | --trunk <trunk-branch-name>]
#         Create a new branch as part of a stack. If parent is not specified,
#         the previous branch in the stack is used (if any). If trunk is
#         specified, the branch is rooted from the given branch.
#         trunk-branch-name can be either a branch name or a branch name with a
#         commit ID in the format "<branch-name>@<commit-id>".
# p, pick <commit-id>
#         Pick a commit to be included in the stack. Only valid after a
#         stack-branch command.

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[ErichKramer]

nit: both are correct

[twavv]

Yep, only changing this because sync is more common and this is the only instance in this codebase where we use synch instead.