rdopkg patch fails if format.coverletter = auto

[yazug]

Had an instance where a user enabled at least the following and when rdopkg patch was run it generated a bad patchset

[format]
signoff = true
coverletter = auto

all of the patches were signed (not a problem just noise) but there was a 0000-cover-letter.patch included that failed to apply and I think there was a hole in the patch set at some point but didn't reproduce any beyond this.

I think the fix for this would be to make sure git format-patch also passes in --no-cover-letter option when it makes the call.

[stephenfin]

It also fails if diff.noprefix is set to True.