Suggestion: delta-exclusion = compress-exclusion

It seems very likely to me that all of the masks used for
compress-exclusion should also be passed in with --exclude-delta-sig as
well.  It might be useful to have a single option that makes this
happen.  I'll add a delta-exclusion option to my darrc.

It might also be useful to exclude read-only files from deltas, but I
don't see how to do that easily.