Add a test for git-config --file

Check for non-0 exit code if the confiog file does not exist and
if it works exactly like when setting GIT_CONFIG.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1300-repo-config.sh

@@ -325,6 +325,9 @@ EOF
 
 test_expect_success 'new variable inserts into proper section' 'cmp .git/config expect'
 
+test_expect_success 'alternative GIT_CONFIG (non-existing file should fail)' \
+	'git config --file non-existing-config -l; test $? != 0'
+
 cat > other-config << EOF
 [ein]
 	bahn = strasse
@@ -338,6 +341,9 @@ GIT_CONFIG=other-config git config -l > output
 
 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'
+
 GIT_CONFIG=other-config git config anwohner.park ausweis
 
 cat > expect << EOF