show-branch: fix segfault when showbranch.default exists

When running "git show-branch" without any parameter in a repository that
has showbranch.default defined, we used to rely on the fact that our
handcrafted option parsing loop never looked at av[0].

The array of default strings had the first real command line argument in
default_arg[0], but the option parser wanted to look at the array starting
at av[1], so we assigned the address of -1th element to av to force the
loop start working from default_arg[0].

This no longer worked since 5734365 (show-branch: migrate to parse-options
API, 2009-05-21), as parse_options_start() saved the incoming &av[0] in
its ctx->out and later in parse_options_end() it did memmove to ctx->out
(with ctx->cpidx == 0), overwriting the memory before default_arg[] array.

I am not sure if this is a bug in parse_options(), or a bug in the caller,
and tonight I do not have enough concentration to figure out which.  In
any case, this patch works the issue around.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-show-branch.c

@@ -565,7 +565,15 @@ static int git_show_branch_config(const char *var, const char *value, void *cb)
 	if (!strcmp(var, "showbranch.default")) {
 		if (!value)
 			return config_error_nonbool(var);
-		if (default_alloc <= default_num + 1) {
+		/*
+		 * default_arg is now passed to parse_options(), so we need to
+		 * mimick the real argv a bit better.
+		 */
+		if (!default_num) {
+			default_alloc = 20;
+			default_arg = xcalloc(default_alloc, sizeof(*default_arg));
+			default_arg[default_num++] = "show-branch";
+		} else if (default_alloc <= default_num + 1) {
 			default_alloc = default_alloc * 3 / 2 + 20;
 			default_arg = xrealloc(default_arg, sizeof *default_arg * default_alloc);
 		}
@@ -693,8 +701,8 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 
 	/* If nothing is specified, try the default first */
 	if (ac == 1 && default_num) {
-		ac = default_num + 1;
-		av = default_arg - 1; /* ick; we would not address av[0] */
+		ac = default_num;
+		av = default_arg;
 	}
 
 	ac = parse_options(ac, av, builtin_show_branch_options,

t/t3202-show-branch-octopus.sh

@@ -56,4 +56,12 @@ test_expect_success 'show-branch with more than 8 branches' '
 
 '
 
+test_expect_success 'show-branch with showbranch.default' '
+	for i in $numbers; do
+		git config --add showbranch.default branch$i
+	done &&
+	git show-branch >out &&
+	test_cmp expect out
+'
+
 test_done