t7800: Test difftool passing arguments to diff

git-difftool relies on the ability to forward unknown arguments
to the git-diff command.  Add a test to ensure that this works
as advertised.

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

t/t7800-difftool.sh

@@ -83,6 +83,17 @@ test_expect_success PERL 'difftool ignores bad --tool values' '
 	test "$diff" = ""
 '
 
+test_expect_success PERL 'difftool forwards arguments to diff' '
+	>for-diff &&
+	git add for-diff &&
+	echo changes>for-diff &&
+	git add for-diff &&
+	diff=$(git difftool --cached --no-prompt -- for-diff) &&
+	test "$diff" = "" &&
+	git reset -- for-diff &&
+	rm for-diff
+'
+
 test_expect_success PERL 'difftool honors --gui' '
 	git config merge.tool bogus-tool &&
 	git config diff.tool bogus-tool &&