diff-opts can no longer be used multiple times

[svenwest]

stg 2.0.0-alpha.0

I use the stgit emacs mode, and it fails to show the diff of a patch with the following error:

error: The argument '--diff-opts <OPTIONS>' was provided more than once, but cannot be used multiple times

It seems rust-stg expects a space-separated string, while legacy-stg expects multiple occurences of the -O argument.

[svenwest]

I tried to create this under the label 'rust' but I can't edit the labels, and there's no label 'rust'.

[jpgrayson]

Resolved in #189.