feat(stack reorder): Delete branches removed from reorder plan

[twavv]

Example

Have a branch structure:

* 2023-06-08-two-b ce20b00  (HEAD -> 2023-06-08-two)
| 
* 2023-06-08-two-a 1f8f16b 
| 
* 2023-06-08-one-b 061ed6e  (2023-06-08-one)
| 
* 2023-06-08-one-a a4db90d 
| 
* 2023-05-30-one-a (#957) 87232f0  (origin/main, origin/HEAD, main)

Run av stack reorder and delete the stack-branch 2023-06-08-two line (effectively merging all commits into 2023-06-08-one):

Choose e to edit the plan:

(looks like unfortunately we lose the comment information, but this can be fixed in a future change)

Save the same plan, choose o to orphan the branches:

Confirmed that 2023-06-08-two still exists and hasn't had it's HEAD modified.

Reset to the state above, but this time choose d to delete the branches:

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[draftcode]

--delete-ref looks confusing? A branch is a ref.

Aside from that, do we want to have this command? It seems that simply deleting a line would be suffice to support deleting branches, and for I cannot think a good use case for this command.

[twavv]

Yeah, I meant it to disambiguate deleting av's version of the branch and deleting the actual Git ref. It could also be --delete-git but I'm not sure if that's actually any clearer. Maybe --delete-git-ref would be the clearest of all.

But to your point, the only reason I implemented this as a command is convenience of implementation. It's nice to be able to just append the DeleteBranch commands to the reorder plan if we detect that the branches have been removed from the plan (which is what we do here). We probably don't even need to document that this exists. We only even need the flag version because the command is serialized as a string whenever we write the continuation to file.