Enhance "git status --porcelain" output by collecting more data on
the state of the index and the working tree files, which may
further be used to teach git-prompt (in contrib/) to make fewer
calls to git.

* jh/status-v2-porcelain:
  status: unit tests for --porcelain=v2
  test-lib-functions.sh: add lf_to_nul helper
  git-status.txt: describe --porcelain=v2 format
  status: print branch info with --porcelain=v2 --branch
  status: print per-file porcelain v2 status data
  status: collect per-file data for --porcelain=v2
  status: support --porcelain[=<version>]
  status: cleanup API to wt_status_print
  status: rename long-format print routines