[MERGE][Take Five] Show the diff in the commit messages

[Martin Pool]

I've addressed these and will send it in.  Thanks Goffredo, Aaron.

> ^^^ It is nicer to make 'utf-8' the default parameter, instead of None.  (unless you need to distinguish between a caller-supplied 'utf-8' and the default 'utf-8')

In general, I think accepting None as unspecified can be good, so that
callers can explicitly ask for the default without knowing what it is.
 That can make it easier to write wrapper functions and so on.  In
this case I don't have a strong opinion either way.

-- 
Martin