Check if pkgbuild merge is possible | resolves #2435

[theaino]

Fixing #2435 Sometimes aur pkgbuild repos don't contain any branches leaving them in a detached head state (like the mhwd-nvidia repo). When this is the case, the package can't be installed. This is fixed by only merging when the repo contains at least one branch.

[Jguer]

Makes sense @AinoSpring, it's an edge case fix but at least it shouldn't break regular installs.

I guess users that have checked out a single commit to build a specific version are "out of warranty" anyway.

EDIT: should just be missing test updates