diff: don't crash with empty argument to -G or -S

The pickaxe options, -G and -S, need either a regex or a string to look
through the history for.  An empty value isn't very useful since it
would either match everything or nothing, and what's worse, we presently
crash with a BUG like so when the user provides one:

    BUG: diffcore-pickaxe.c:241: should have needle under -G or -S

Since it's not very nice of us to crash and this wouldn't do anything
useful anyway, let's simply inform the user that they must provide a
non-empty argument and exit with an error if they provide an empty one
instead.

Reported-by: Jared Van Bortel <cebtenzzre@gmail.com>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -5493,6 +5493,8 @@ static int diff_opt_pickaxe_regex(const struct option *opt,
 	BUG_ON_OPT_NEG(unset);
 	options->pickaxe = arg;
 	options->pickaxe_opts |= DIFF_PICKAXE_KIND_G;
+	if (arg && !*arg)
+		return error(_("-G requires a non-empty argument"));
 	return 0;
 }
 
@@ -5504,6 +5506,8 @@ static int diff_opt_pickaxe_string(const struct option *opt,
 	BUG_ON_OPT_NEG(unset);
 	options->pickaxe = arg;
 	options->pickaxe_opts |= DIFF_PICKAXE_KIND_S;
+	if (arg && !*arg)
+		return error(_("-S requires a non-empty argument"));
 	return 0;
 }
 

t/t4209-log-pickaxe.sh

@@ -93,6 +93,22 @@ test_expect_success 'usage: --no-pickaxe-regex' '
 	test_cmp expect actual
 '
 
+test_expect_success 'usage: -G and -S with empty argument' '
+	cat >expect <<-\EOF &&
+	error: -S requires a non-empty argument
+	EOF
+
+	test_expect_code 129 git log -S "" 2>actual &&
+	test_cmp expect actual &&
+
+	cat >expect <<-\EOF &&
+	error: -G requires a non-empty argument
+	EOF
+
+	test_expect_code 129 git log -G "" 2>actual &&
+	test_cmp expect actual
+'
+
 test_log	expect_initial	--grep initial
 test_log	expect_nomatch	--grep InItial
 test_log_icase	expect_initial	--grep InItial