difftool / mergetool waiting

Hello,

I would like an option to difftool and mergetool that would control
waiting for a return code from the difftool: wait.


Like prompt, wait would be configurable with the difftool.name.

Like prompt and for backwards compatibility, wait would default true.


If wait is false, prompt would be overridden to be false.

If wait is false, git would fire off the tool command and proceed to
the next then exit cleanly.

If wait is true, git would behave as now: it will wait for an exit
code of each tool execution before proceeding.


Thank you,

Jonathan