setup_git_env: convert die("BUG") to BUG()

Converting to BUG() makes it easier to detect and debug
cases where we hit this assertion. Coupled with a new test
in t1300, this shows that the test suite can detect such
corner cases.

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

environment.c

@@ -169,7 +169,7 @@ static void setup_git_env(void)
 	git_dir = getenv(GIT_DIR_ENVIRONMENT);
 	if (!git_dir) {
 		if (!startup_info->have_repository)
-			die("BUG: setup_git_env called without repository");
+			BUG("setup_git_env called without repository");
 		git_dir = DEFAULT_GIT_DIR_ENVIRONMENT;
 	}
 	gitfile = read_gitfile(git_dir);

t/t1300-repo-config.sh

@@ -1539,4 +1539,10 @@ test_expect_success !MINGW '--show-origin blob ref' '
 	test_cmp expect output
 '
 
+test_expect_failure '--local requires a repo' '
+	# we expect 128 to ensure that we do not simply
+	# fail to find anything and return code "1"
+	test_expect_code 128 nongit git config --local foo.bar
+'
+
 test_done