GSS-Negotiate authentication requires that all data fit into postbuffer

One thing I just noticed is that if git is using GSS-Negotiate authentication, the entire POST contents have to fit into however much memory is specified by http.postbuffer:

  vauxhall ok % git push https://bmc@git.crustytoothpaste.net/git/bmc/test.git development
  Counting objects: 37994, done.
  Delta compression using up to 4 threads.
  Compressing objects: 100% (10683/10683), done.
  Writing objects: 100% (37994/37994), 9.15 MiB | 4.45 MiB/s, done.
  Total 37994 (delta 26760), reused 37633 (delta 26467)
  Unable to rewind rpc post data - try increasing http.postBuffer
  Password for 'https://bmc@git.crustytoothpaste.net': 

GSS-Negotiate authentication always requires a rewind with CURL.

The remote in question only supports Negotiate authentication, so prompting for a password in this case isn't going to help. I'm probably going to look into this over the next couple of days, but two things need to be done here: 1) do not prompt for a password if only Negotiate authentication is requested, since it just won't work, and 2) recreate the data as needed if we have to rewind, since otherwise pushing a fresh copy of the Linux kernel repo simply isn't going to work as the buffer will have to be too large. An alternative is to send a small amount of data, smaller than the postbuffer, in the first chunk and only fail to rewind if the second or subsequent chunks need rewinding.