gitproto: document the orig-path keying choice on challengeURLFor

Path-aware credential helpers (credential.useHttpPath=true) interact with our redirect-handling in a subtle way: we key the helper Lookup on the user-typed path, not the redirect target's path, so creds stored against the URL the user configured stay findable when the redirect rewrites paths. The trade-off is a slightly imprecise Approve/Reject key for path-aware helpers — not a credential leak (the creds still only reach hosts the user trusted) but worth calling out so a future maintainer doesn't "fix" the behaviour the other way and break the common case.

Doc-only.