git: Parse delim-pkt and response-end-pkt
Git Protocol version 2 defines 0x0001 as a delimiter packet that separates the sections of a message, as well as 0x0002 as a response_end packet that indicates the end of a response for stateless connections.
Add parsing and checking of the delim-pkt and response_end-pkt lines, adding them as items to the tree for ease of reading and filtering while handling pre-existing "malformed" errors. For additional consistency, the terminator 0x0000 is now referred to as Flush packet.
Part of #17093