hook: add --porcelain to list command

Teach 'git hook list --porcelain <hookname>', which prints simply the
commands to be run in the order suggested by the config. This option is
intended for use by user scripts, wrappers, or out-of-process Git
commands which still want to execute hooks. For example, the following
snippet might be added to git-send-email.perl to introduce a
`pre-send-email` hook:

  sub pre_send_email {
    open(my $fh, 'git hook list --porcelain pre-send-email |');
    chomp(my @hooks = <$fh>);
    close($fh);

    foreach $hook (@hooks) {
            system $hook
    }

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-hook.txt

@@ -8,7 +8,7 @@ git-hook - Manage configured hooks
 SYNOPSIS
 --------
 [verse]
-'git hook' list <hook-name>
+'git hook' list [--porcelain] <hook-name>
 
 DESCRIPTION
 -----------
@@ -43,11 +43,20 @@ Local config
 COMMANDS
 --------
 
-list <hook-name>::
+list [--porcelain] <hook-name>::
 
 List the hooks which have been configured for <hook-name>. Hooks appear
 in the order they should be run, and note the config scope where the relevant
 `hook.<hook-name>.command` was specified, not the `hookcmd` (if applicable).
++
+If `--porcelain` is specified, instead print the commands alone, separated by
+newlines, for easy parsing by a script.
+
+OPTIONS
+-------
+--porcelain::
+	With `list`, print the commands in the order they should be run,
+	separated by newlines, for easy parsing by a script.
 
 GIT
 ---

builtin/hook.c

@@ -16,8 +16,11 @@ static int list(int argc, const char **argv, const char *prefix)
 	struct list_head *head, *pos;
 	struct hook *item;
 	struct strbuf hookname = STRBUF_INIT;
+	int porcelain = 0;
 
 	struct option list_options[] = {
+		OPT_BOOL(0, "porcelain", &porcelain,
+			 "format for execution by a script"),
 		OPT_END(),
 	};
 
@@ -29,6 +32,8 @@ static int list(int argc, const char **argv, const char *prefix)
 			      builtin_hook_usage, list_options);
 	}
 
+
+
 	strbuf_addstr(&hookname, argv[0]);
 
 	head = hook_list(&hookname);
@@ -41,10 +46,14 @@ static int list(int argc, const char **argv, const char *prefix)
 
 	list_for_each(pos, head) {
 		item = list_entry(pos, struct hook, list);
-		if (item)
-			printf("%s:\t%s\n",
-			       config_scope_name(item->origin),
-			       item->command.buf);
+		if (item) {
+			if (porcelain)
+				printf("%s\n", item->command.buf);
+			else
+				printf("%s:\t%s\n",
+				       config_scope_name(item->origin),
+				       item->command.buf);
+		}
 	}
 
 	clear_hook_list();

t/t1360-config-based-hooks.sh

@@ -55,4 +55,15 @@ test_expect_success 'git hook list reorders on duplicate commands' '
 	test_cmp expected actual
 '
 
+test_expect_success 'git hook list --porcelain prints just the command' '
+	cat >expected <<-\EOF &&
+	/path/ghi
+	/path/abc
+	/path/def
+	EOF
+
+	git hook list --porcelain pre-commit >actual &&
+	test_cmp expected actual
+'
+
 test_done