parse-options: fix SunCC compiler warning

The compiler reports this because show_gitcomp() never actually
returns a value:

    "parse-options.c", line 520: warning: Function has no return
    statement : show_gitcomp

We could shut the compiler up. But instead let's not bury exit() too
deep. Do the same as internal -h handling, return a special error code
and handle the exit() in parse_options() (and other
parse_options_step() callers) instead.

Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pclouds

builtin/blame.c

@@ -844,6 +844,8 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 		case PARSE_OPT_HELP:
 		case PARSE_OPT_ERROR:
 			exit(129);
+		case PARSE_OPT_COMPLETE:
+			exit(0);
 		case PARSE_OPT_DONE:
 			if (ctx.argv[0])
 				dashdash_pos = ctx.cpidx;

builtin/shortlog.c

@@ -286,6 +286,8 @@ int cmd_shortlog(int argc, const char **argv, const char *prefix)
 		case PARSE_OPT_HELP:
 		case PARSE_OPT_ERROR:
 			exit(129);
+		case PARSE_OPT_COMPLETE:
+			exit(0);
 		case PARSE_OPT_DONE:
 			goto parse_done;
 		}

builtin/update-index.c

@@ -1071,6 +1071,8 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 		case PARSE_OPT_HELP:
 		case PARSE_OPT_ERROR:
 			exit(129);
+		case PARSE_OPT_COMPLETE:
+			exit(0);
 		case PARSE_OPT_NON_OPTION:
 		case PARSE_OPT_DONE:
 		{

parse-options.c

@@ -516,7 +516,7 @@ static int show_gitcomp(struct parse_opt_ctx_t *ctx,
 	show_negated_gitcomp(original_opts, -1);
 	show_negated_gitcomp(original_opts, nr_noopts);
 	fputc('\n', stdout);
-	exit(0);
+	return PARSE_OPT_COMPLETE;
 }
 
 static int usage_with_options_internal(struct parse_opt_ctx_t *,
@@ -638,6 +638,8 @@ int parse_options(int argc, const char **argv, const char *prefix,
 	case PARSE_OPT_HELP:
 	case PARSE_OPT_ERROR:
 		exit(129);
+	case PARSE_OPT_COMPLETE:
+		exit(0);
 	case PARSE_OPT_NON_OPTION:
 	case PARSE_OPT_DONE:
 		break;

parse-options.h

@@ -197,6 +197,7 @@ extern int opterror(const struct option *opt, const char *reason, int flags);
 /*----- incremental advanced APIs -----*/
 
 enum {
+	PARSE_OPT_COMPLETE = -2,
 	PARSE_OPT_HELP = -1,
 	PARSE_OPT_DONE,
 	PARSE_OPT_NON_OPTION,