t7800-difftool: fix the effectless GIT_DIFFTOOL_PROMPT test

GIT_DIFFTOOL_PROMPT doesn't have any effect if overridden with --prompt.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index ebdccf9..fff6a6d 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -136,7 +136,7 @@
 	GIT_DIFFTOOL_PROMPT=true &&
 	export GIT_DIFFTOOL_PROMPT &&
 
-	prompt=$(echo | git difftool --prompt branch | tail -1) &&
+	prompt=$(echo | git difftool branch | tail -1) &&
 	prompt_given "$prompt" &&
 
 	restore_test_defaults