difftool: don't assume that default sh is sane

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 <redacted>
Helped-by: John Keeping <redacted>
Signed-off-by: David Aguilar <redacted>
Signed-off-by: Junio C Hamano <redacted>