difftool: don't assume that default sh is sane

git-difftool used to create a command list script containing $( ... )
and explicitly calls "sh -c" with this list.

Instead, allow mergetool --tool-help to take a mode parameter and call
mergetool directly to invoke the show_tool_help function. This mode
parameter is intented for use solely by difftool.

Signed-off-by: Charles Bailey <cbailey32@bloomberg.net>
Helped-by: John Keeping <john@keeping.me.uk>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool.sh

@@ -326,6 +326,10 @@ prompt=$(git config --bool mergetool.prompt || echo true)
 while test $# != 0
 do
 	case "$1" in
+	--tool-help=*)
+		TOOL_MODE=${1#--tool-help=}
+		show_tool_help
+		;;
 	--tool-help)
 		show_tool_help
 		;;