Split diffOpts and logOpts settings using shlex.split().

The two options diffOpts and logOpts are command-line options.
Splitting them into words on whitespace was too crude, because
individual command-line options might need to include spaces; e.g.,

    [multimailhook]
            logOpts = --pretty=format:"%h %aN <%aE>%n%s%n%n%b%n"

So instead use shlex.split(), which understands quoting etc.
Clarify documentation of logOpts wrt shell quoting