Improve the stack branch command to handle trunk branches as parents

[draftcode]

When creating a new branch from a trunk branch, the stack branch command
is better to start off from the remote tracking branch instead of the
local branch. This way, the new branch will be created from the latest
state of the trunk branch.

While we are here, accept origin/HEAD as the parent branch name and
resolve it to the default branch.

Previously, this command automatically "adopts" the parent branch to av
automatically, assuming that the parent branch's parent is the default
branch, which might not be true. Instead, this command checks if the
parent branch is already adopted or not.

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Stack

1. 👉 Improve the stack branch command to handle trunk branches as parents #344 👈 (this pr)

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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: 0)

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

Detailed Breakdown

Author’s expertise score for the modified files:

• cmd/av/stack_branch.go (1.00)
• e2e_tests/stack_orphan_test.go (1.00)
• e2e_tests/stack_restack_test.go (1.00)
• e2e_tests/stack_sync_amend_test.go (1.00)
• e2e_tests/stack_sync_delete_merged_test.go (1.00)
• e2e_tests/stack_sync_delete_parent_test.go (1.00)
• e2e_tests/stack_sync_merge_commit_test.go (1.00)
• e2e_tests/stack_sync_merged_parent_test.go (1.00)
• e2e_tests/stack_sync_test.go (1.00)
• e2e_tests/stack_sync_trunk_test.go (1.00)

Files	Reviewers
cmd/av/stack_branch.go †, e2e_tests/stack_orphan_test.go †, e2e_tests/stack_restack_test.go †, e2e_tests/stack_sync_amend_test.go †, e2e_tests/stack_sync_delete_merged_test.go †, e2e_tests/stack_sync_delete_parent_test.go †, e2e_tests/stack_sync_merge_commit_test.go †, e2e_tests/stack_sync_merged_parent_test.go †, e2e_tests/stack_sync_test.go †, e2e_tests/stack_sync_trunk_test.go †	@jainankit (score: 0.86, current review load: 0)

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

See full breakdown of the reviewers on the Aviator webapp.

[jainankit]

A few questions (user experience related):

• With this new functionality, would we internally do a git fetch for that remote ref in case our local trunk branch is not up to date?
• A separate minor UX note that now the behavior is slightly different for trunk branch vs feature branches. We should show this as a message to the user when running av stack branch on trunk.
• If we report an error when parent is not adopted, we should recommend user with the command to adopt the entire stack. There could be scenario where the user wants to push multiple branches together. Or there could be a param within av stack branch to support that behavior.

[draftcode]

A few questions (user experience related):

• With this new functionality, would we internally do a git fetch for that remote ref in case our local trunk branch is not up to date?

Answered offline, but this is using the remote tracking branches, which is a local ref. It doesn't do any git-fetch.

• A separate minor UX note that now the behavior is slightly different for trunk branch vs feature branches. We should show this as a message to the user when running av stack branch on trunk.

This is consistent with av-stack-sync behavior.

• If we report an error when parent is not adopted, we should recommend user with the command to adopt the entire stack. There could be scenario where the user wants to push multiple branches together. Or there could be a param within av stack branch to support that behavior.

I will update the error message.