Add new sequencer object

[draftcode]

This is similar to git's sequencer, but it's for re-stacking branches.

https://github.com/git/git/blob/master/sequencer.h

This object is responsible for re-stacking branches based on the planned
operations.

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Current Aviator status

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Stack

1. 👉 Add new sequencer object #288 👈 (this pr)
2. Add restack command #293

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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:

• internal/git/git.go (1.00)
• internal/sequencer/planner/planner.go (0.00)
• internal/sequencer/planner/targets.go (0.00)
• internal/sequencer/sequencer.go (0.00)

Files	Reviewers
internal/git/git.go †	@jainankit (score: 0.85, 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 #287 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 new_sync
# 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

[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 new_sync
# 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