status: add --porcelain output format

status: add --porcelain output format

The "short" format was added to "git status" recently to
provide a less verbose way of looking at the same
information. This has two practical uses:

  1. Users who want a more dense display of the information.

  2. Scripts which want to parse the information and need a
     stable, easy-to-parse interface.

For now, the "--short" format covers both of those uses.
However, as time goes on, users of (1) may want additional
format tweaks, or for "git status" to change its behavior
based on configuration variables. Those wishes will be at
odds with (2), which wants to stability for scripts.

This patch introduces a separate --porcelain option early to
avoid problems later on.  Right now the --short and
--porcelain outputs are identical. However, as time goes on,
we will have the freedom to customize --short for human
consumption while keeping --porcelain stable.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>