git-remote-entire: amend the agent on the command line, not the first chunk

A shallow push sends shallow <oid> lines before the first command, and send-pack's stateless-rpc framing can split that command across 65515-byte packets. Amending only the first chunk left the helper agent off every connect-path push from a shallow clone, and failed v2 pushes with enough grafts to push the command past the first packet.

Walk the pkt-lines across chunks to the first line carrying a NUL, copy and amend only that line, and re-slice the chunks around it so the pack stays where it is.