manipulating repository metadata

[iurly]

Hi @xolox,
I have a very specific need (TBH I expected this to be quite common but turns out it's not).
I have a debian repository hosted on Amazon S3, and I would like the ability to upload new packages (more specifically, new versions of a package) without re-creating all the repo metadata from .deb files all over again.
So the idea would be to either extend repo.update_repository() with a flag to read the existing Packages file beforehand, or somehow add the possibility to merge two existing repositories.

Would you accept a PR for this kind of feature?