t4068: add --merge-base tests

In the future, we will be adding more --merge-base tests to this test
script. To prepare for that, rename the script accordingly and update
its description. Also, add two basic --merge-base tests that don't
require any functionality to be implemented yet.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4068-diff-symmetric.sh → t/t4068-diff-symmetric-merge-base.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-test_description='behavior of diff with symmetric-diff setups'
+test_description='behavior of diff with symmetric-diff setups and --merge-base'
 
 . ./test-lib.sh
 
@@ -88,4 +88,13 @@ test_expect_success 'diff with ranges and extra arg' '
 	test_i18ngrep "usage" err
 '
 
+test_expect_success 'diff --merge-base with no commits' '
+	test_must_fail git diff --merge-base
+'
+
+test_expect_success 'diff --merge-base with three commits' '
+	test_must_fail git diff --merge-base br1 br2 master 2>err &&
+	test_i18ngrep "usage" err
+'
+
 test_done