interpret-trailers: add --parse convenience option

The last few commits have added command line options that
can turn interpret-trailers into a parsing tool. Since
they'd most often be used together, let's provide a
convenient single option for callers to invoke this mode.

This is implemented as a callback rather than a boolean so
that its effect is applied immediately, as if those options
had been specified. Later options can then override them.
E.g.:

  git interpret-trailers --parse --no-unfold

would work.

Let's also update the documentation to make clear that this
parsing mode behaves quite differently than the normal
"add trailers to the input" mode.

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

Documentation/git-interpret-trailers.txt

@@ -3,24 +3,27 @@ git-interpret-trailers(1)
 
 NAME
 ----
-git-interpret-trailers - help add structured information into commit messages
+git-interpret-trailers - add or parse structured information in commit messages
 
 SYNOPSIS
 --------
 [verse]
-'git interpret-trailers' [--in-place] [--trim-empty] [(--trailer <token>[(=|:)<value>])...] [<file>...]
+'git interpret-trailers' [options] [(--trailer <token>[(=|:)<value>])...] [<file>...]
+'git interpret-trailers' [options] [--parse] [<file>...]
 
 DESCRIPTION
 -----------
-Help adding 'trailers' lines, that look similar to RFC 822 e-mail
+Help parsing or adding 'trailers' lines, that look similar to RFC 822 e-mail
 headers, at the end of the otherwise free-form part of a commit
 message.
 
 This command reads some patches or commit messages from either the
-<file> arguments or the standard input if no <file> is specified. Then
+<file> arguments or the standard input if no <file> is specified. If
-this command applies the arguments passed using the `--trailer`
+`--parse` is specified, the output consists of the parsed trailers.
-option, if any, to the commit message part of each input file. The
+
-result is emitted on the standard output.
+Otherwise, the this command applies the arguments passed using the
+`--trailer` option, if any, to the commit message part of each input
+file. The result is emitted on the standard output.
 
 Some configuration variables control the way the `--trailer` arguments
 are applied to each commit message and the way any existing trailer in
@@ -92,6 +95,10 @@ OPTIONS
 	Remove any whitespace-continuation in trailers, so that each
 	trailer appears on a line by itself with its full content.
 
+--parse::
+	A convenience alias for `--only-trailers --only-input
+	--unfold`.
+
 CONFIGURATION VARIABLES
 -----------------------
 

builtin/interpret-trailers.c

@@ -16,6 +16,16 @@ static const char * const git_interpret_trailers_usage[] = {
 	NULL
 };
 
+static int parse_opt_parse(const struct option *opt, const char *arg,
+			   int unset)
+{
+	struct process_trailer_options *v = opt->value;
+	v->only_trailers = 1;
+	v->only_input = 1;
+	v->unfold = 1;
+	return 0;
+}
+
 int cmd_interpret_trailers(int argc, const char **argv, const char *prefix)
 {
 	struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
@@ -27,6 +37,8 @@ int cmd_interpret_trailers(int argc, const char **argv, const char *prefix)
 		OPT_BOOL(0, "only-trailers", &opts.only_trailers, N_("output only the trailers")),
 		OPT_BOOL(0, "only-input", &opts.only_input, N_("do not apply config rules")),
 		OPT_BOOL(0, "unfold", &opts.unfold, N_("join whitespace-continued values")),
+		{ OPTION_CALLBACK, 0, "parse", &opts, NULL, N_("set parsing options"),
+			PARSE_OPT_NOARG | PARSE_OPT_NONEG, parse_opt_parse },
 		OPT_STRING_LIST(0, "trailer", &trailers, N_("trailer"),
 				N_("trailer(s) to add")),
 		OPT_END()