Broken packages are not correctly detected

[kit-ty-kate]

Camelus uses the branch from where PR are made from without merging it in master first when it checks for broken packages. Which for some people that don't use opam-repository that much can be a big problem.

For example this PR has one commit on top of a commit made in 2017: ocaml/opam-repository#16158. This example is not the first one I've encountered, it's the first time I bother opening an issue about it. There are plenty of similar examples to take from.

[thomasblanc]

@AltGr I guess you can assign me to this :-)