Add restack command

[draftcode]

This uses the new sequencer object to restack the branches.

[aviator-app]

Current Aviator status

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Stack

1. 👉 Add restack command #290 👈 (this pr)

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[aviator-app]

FlexReview Summary

Based on the code complexity and the author's expertise score, these are the suggested reviewers:

• @jainankit (current review load: 4)

See the list of alternate reviewers in the detailed breakdown below.

Detailed Breakdown

Author’s expertise score for the modified files:

• cmd/av/stack.go (1.00)
• cmd/av/stack_restack.go (1.00)
• cmd/av/stack_switch.go (1.00)
• cmd/av/stack_tree.go (1.00)
• go.mod (1.00)
• go.sum (1.00)
• internal/actions/pr.go (1.00)
• internal/actions/sync_branch.go (1.00)
• internal/git/state_file.go (1.00)
• internal/sequencer/planner/planner.go (0.00)
• internal/sequencer/sequencer.go (0.00)
• internal/utils/stackutils/stackutils.go (1.00)

Files	Reviewers
cmd/av/stack.go †, cmd/av/stack_restack.go †, cmd/av/stack_switch.go †, cmd/av/stack_tree.go †, go.mod †, go.sum †, internal/actions/pr.go †, internal/actions/sync_branch.go †, internal/git/state_file.go †, internal/utils/stackutils/stackutils.go †	@jainankit (score: 0.61, current review load: 4)

† Indicates that the file doesn't need an expert review. (?)

See full breakdown of the reviewers on the Aviator webapp.

[aviator-app]

This PR is stacked on top of #289 which was merged. This PR (and its dependents) need to be synchronized on the commit that was merged into master. From your repository, run the following commands:

# Switch to this branch
git checkout restack
# Sync this branch (and its children) on top of the merge commit
av stack sync
# Optionally update pull requests to match new stack
av stack submit