builtin-config: Fix crash when using "-f <relative path>" from non-root dir

When your current directory is not at the root of the working tree, and you
use the "-f" option with a relative path, the current code tries to read
from a wrong file, since argv[2] is now beyond the end of the rearranged
argument list.

This patch replaces the incorrect argv[2] with the variable holding the
given config file name.

The bug was introduced by d64ec16 (git config: reorganize to use parseopt).

[jc: added test]

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-config.c

@@ -339,7 +339,7 @@ int cmd_config(int argc, const char **argv, const char *unused_prefix)
 		if (!is_absolute_path(given_config_file) && prefix)
 			config_exclusive_filename = prefix_filename(prefix,
 								    strlen(prefix),
-								    argv[2]);
+								    given_config_file);
 		else
 			config_exclusive_filename = given_config_file;
 	}

t/t1300-repo-config.sh

@@ -398,6 +398,17 @@ test_expect_success 'alternative GIT_CONFIG' 'cmp output expect'
 test_expect_success 'alternative GIT_CONFIG (--file)' \
 	'git config --file other-config -l > output && cmp output expect'
 
+test_expect_success 'refer config from subdirectory' '
+	mkdir x &&
+	(
+		cd x &&
+		echo strasse >expect
+		git config --get --file ../other-config ein.bahn >actual &&
+		test_cmp expect actual
+	)
+
+'
+
 GIT_CONFIG=other-config git config anwohner.park ausweis
 
 cat > expect << EOF