Try checkout the default branch if the current branch is deleted

[draftcode]

This is a continuation of #357. There are people who prefers checkout
the default branch if the current branch is deleted after av stack sync.
All things considered, change the behavior like this. After av stack
sync:

1. If the originally checked out branch still exists, check it out.
2. If the originally checked out branch is deleted, check out the
   default branch if it exists.
3. If the default branch doesn't exist, check out the remote tracking
   branch of the default branch if it exists. (It should exist at this
   point usually.)
4. Otherwise, do nothing.

This should satisfy all the parties.

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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:

• internal/git/gitui/prune.go (1.00)

Files	Reviewers
internal/git/gitui/prune.go †	@jainankit (score: 0.69, current review load: 0)

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

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