t7800-difftool.sh: Test mergetool.prompt fallback

4cacc621 made difftool fall back to mergetool.prompt
when difftool.prompt is unconfigured.  This adds a test.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7800-difftool.sh

@@ -27,6 +27,7 @@ remove_config_vars()
 	git config --unset difftool.prompt
 	git config --unset merge.tool
 	git config --unset mergetool.test-tool.cmd
+	git config --unset mergetool.prompt
 	return 0
 }
 
@@ -159,6 +160,17 @@ test_expect_success 'difftool.prompt config variable is false' '
 	restore_test_defaults
 '
 
+# Test that we don't have to pass --no-prompt when mergetool.prompt is false
+test_expect_success 'difftool merge.prompt = false' '
+	git config --unset difftool.prompt
+	git config mergetool.prompt false &&
+
+	diff=$(git difftool branch) &&
+	test "$diff" = "branch" &&
+
+	restore_test_defaults
+'
+
 # Test that the -y flag can override difftool.prompt = true
 test_expect_success 'difftool.prompt can overridden with -y' '
 	git config difftool.prompt true &&