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