Make difftool.prompt fall back to mergetool.prompt

The documentation states that "git-difftool falls back to git-mergetool
config variables when the difftool equivalents have not been defined".
Until now, this was not the case for "difftool.prompt".

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

git-difftool--helper.sh

@@ -11,7 +11,8 @@ TOOL_MODE=diff
 # difftool.prompt controls the default prompt/no-prompt behavior
 # and is overridden with $GIT_DIFFTOOL*_PROMPT.
 should_prompt () {
-	prompt=$(git config --bool difftool.prompt || echo true)
+	prompt_merge=$(git config --bool mergetool.prompt || echo true)
+	prompt=$(git config --bool difftool.prompt || echo $prompt_merge)
 	if test "$prompt" = true; then
 		test -z "$GIT_DIFFTOOL_NO_PROMPT"
 	else