fix(git): preserve branch all remote refs

[sjh9714]

Summary

Fixes #2137.

• Preserve native git branch -a / git branch --all output so explicit all-branch listings keep remote-tracking refs such as remotes/origin/main.
• Keep the default compact rtk git branch behavior unchanged.
• Add regression coverage for explicit all output and default compact deduping.

Test plan

• cargo fmt --all --check
• cargo test format_branch_output
• cargo test filter_branch
• cargo clippy --all-targets
• cargo test
• Manual testing: rtk git branch -a output inspected in a temp repo with origin/main and a remote-only branch

Note: I used Codex while preparing this change, and I reviewed the final diff and ran the listed checks locally.

[CLAassistant]

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