perf(repo-brancher): skip already-synced branches via upfront ls-remote

[petr-muller]

Summary

• Run a single git ls-remote per repo upfront to get all remote ref SHAs
• Compare current branch SHA against each future branch SHA: if they match, the branch is already synced and all git operations (init, fetch, push) are skipped entirely
• Only enter the expensive clone + push path for branches that actually need work

Stacked on #5014 (merged).

Benchmark (20 repos, 2026-03-19)

Comparison against #5014 (--deepen + --unshallow):

Scenario	Without ls-remote opt	With ls-remote opt	Improvement
Sync (with work)	450s	448s	~same
No-op (reconciled)	42s	7s	83% faster (6x)

The big win is in the no-op case: synced repos skip git init + git fetch + git push entirely, just doing one ls-remote per repo.

🤖 Generated with Claude Code

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This pull-request has been approved by: petr-muller
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