Supports branch name completion for --parent options

[Konboi]

Addressed https://github.com/aviator-co/av/issues/285

I confirmed it locally like the below ref: https://github.com/spf13/cobra/blob/main/site/content/completions/_index.md#debugging-1

$  go build -o av ./cmd/av/
$  ./av __complete stack branch --parent ""
issue-244-br-1
test-2
issue-244-test
new-one
issue-244
new-1
test-1
fuga-11
issue-236
:0
Completion ended with directive: ShellCompDirectiveDefault

$ ./av __complete stack sync --parent ""
fuga-11
issue-236
test-1
test-2
new-one
issue-244
new-1
issue-244-test
issue-244-br-1
:0
Completion ended with directive: ShellCompDirectiveDefault

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FlexReview Summary

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

• @draftcode (current review load: 2)

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 (0.00)
• cmd/av/stack_sync.go (0.00)

Files	Reviewers
cmd/av/stack_branch.go †	@draftcode (score: 1.00, current review load: 2), @doratzeng (score: 0.03, current review load: 2), @jainankit (score: 0.03, current review load: 9)

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