Check if an update is in staging already

[Mic92]

Gdb was merged in staging:
NixOS/nixpkgs#44564
and the bot tried to merge it into master as well:
NixOS/nixpkgs#45235
NixOS/nixpkgs#44780

This probably does not happen too often, but it would be nice to have to detect this at some point.

[jtojnar]

It already does check: https://github.com/ryantm/nixpkgs-update/blob/15f16444caed45b7da82f6b453c8923fa9340437/src/Update.hs#L144-L149. not sure what went wrong.

[ryantm]

I only update the branches every hour or so, but this is off by about 11 days, so probably something is wrong. I'll try to look into what's going on.

[ryantm]

Another issue like this: NixOS/nixpkgs#50738

[ryantm]

Another instance of the issue NixOS/nixpkgs#50821

[ryantm]

NixOS/nixpkgs#50827 similar issue

[ryantm]

Work on this will be related to #91

[ryantm]

Example of problem NixOS/nixpkgs#53345

[ryantm]

Yay, I finally figured out the cause of this bug! The issue is that when it checks for the old version being on staging or master, it only does a sub-string check. So in the case of gdb, it checked if 8.2 is in the derivation file, and of course it is because 8.2 is a sub-string of 8.2.1.