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>

Documentation/git-status.txt

@@ -27,6 +27,11 @@ OPTIONS
 --short::
 	Give the output in the short-format.
 
+--porcelain::
+	Give the output in a stable, easy-to-parse format for scripts.
+	Currently this is identical to --short output, but is guaranteed
+	not to change in the future, making it safe for scripts.
+
 -u[<mode>]::
 --untracked-files[=<mode>]::
 	Show untracked files (Default: 'all').
@@ -45,8 +50,8 @@ used to change the default for when the option is not
 specified.
 
 -z::
-	Terminate entries with NUL, instead of LF.  This implies `-s`
-	(short status) output format.
+	Terminate entries with NUL, instead of LF.  This implies
+	the `--porcelain` output format if no other format is given.
 
 
 OUTPUT

builtin-commit.c

@@ -995,12 +995,16 @@ int cmd_status(int argc, const char **argv, const char *prefix)
 	static enum {
 		STATUS_FORMAT_LONG,
 		STATUS_FORMAT_SHORT,
+		STATUS_FORMAT_PORCELAIN,
 	} status_format = STATUS_FORMAT_LONG;
 	unsigned char sha1[20];
 	static struct option builtin_status_options[] = {
 		OPT__VERBOSE(&verbose),
 		OPT_SET_INT('s', "short", &status_format,
 			    "show status concisely", STATUS_FORMAT_SHORT),
+		OPT_SET_INT(0, "porcelain", &status_format,
+			    "show porcelain output format",
+			    STATUS_FORMAT_PORCELAIN),
 		OPT_BOOLEAN('z', "null", &null_termination,
 			    "terminate entries with NUL"),
 		{ OPTION_STRING, 'u', "untracked-files", &untracked_files_arg,
@@ -1011,7 +1015,7 @@ int cmd_status(int argc, const char **argv, const char *prefix)
 	};
 
 	if (null_termination && status_format == STATUS_FORMAT_LONG)
-		status_format = STATUS_FORMAT_SHORT;
+		status_format = STATUS_FORMAT_PORCELAIN;
 
 	wt_status_prepare(&s);
 	git_config(git_status_config, &s);
@@ -1032,6 +1036,9 @@ int cmd_status(int argc, const char **argv, const char *prefix)
 	case STATUS_FORMAT_SHORT:
 		short_print(&s, null_termination);
 		break;
+	case STATUS_FORMAT_PORCELAIN:
+		short_print(&s, null_termination);
+		break;
 	case STATUS_FORMAT_LONG:
 		s.verbose = verbose;
 		if (s.relative_paths)