ghproxy: support git ref paths with slash-separated branch names

[valen-mascarenhas14]

Fixes: #760

What this PR does ?

This PR fixes ghproxy's path simplifier to correctly handle git reference paths where branch names contain forward slashes, such as feature/v1beta1-migrate or release/v2.0.

Previously, these paths would trigger "Path not handled" debug logs because the simplifier only matched a single path segment after refs/heads/, causing paths like /repos/kubernetes-sigs/agent-sandbox/git/refs/heads/feature/v1beta1-migrate to be unmatched.

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: valen-mascarenhas14
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[kubernetes-prow]

Hi @valen-mascarenhas14. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[petr-muller]

/ok-to-test