Separate apt::pin for apt::backports to allow pin by release instead of ...

[riconnon]

Currently apt::backports creates a ineffective pin when using debian 7 because backports and the main repo share an origin. Pinning by release can, for this reason, be more reliable for backports.

[underscorgan]

@irconan thanks for the contribution! In general this looks good to me, though I'd like @daenney to confirm this looks reasonable. Also, we'd like to have this squashed down to a single commit.

[riconnon]

I've squashed these commits for you. Let me know if you see anything else you'd like changing.

[daenney]

This does seem like a better idea but it does mean we're changing behaviour for users under the hood. Thankfully we're still managing the same file in the end so it'll just update the current pin instead of creating two conflicting ones.

One more reason to remove apt::backports in its entirety, there's no way to do this right by everyone.

Overall I'm 👍 on this.

[underscorgan]

Thanks for the contribution @irconan , and thanks for the feedback @daenney !

[raoulbhatia]

Isn't this basically what i did in #313 ? ;)

Well,
glad that this functionality is merged now!