Please support source:Synopsis subsvar in package synopsis

Hi,

I've been using the fairly new ${source:Synopsis} and ${source:Extended-Description} substvars in my packages to avoid having to do much copy&paste (and run the risk of mistakes).

mentors.debian.net seems to not know about these yet as a recent upload of mine shows the Synopsis substvars syntax instead of the replacements on the webinterface.

cf. https://mentors.debian.net/package/ifupdown-ng/

Thanks, --Daniel

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